Fix typos in gcmodule.c and restructure comments for clarity (GH-17983)

Modules/gcmodule.c

@@ -609,7 +609,7 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
             // NEXT_MASK_UNREACHABLE flag, we set it unconditionally.
             // But this may pollute the unreachable list head's 'next' pointer
             // too. That's semantically senseless but expedient here - the
-            // damage is repaired when this fumction ends.
+            // damage is repaired when this function ends.
             last->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)gc);
             _PyGCHead_SET_PREV(gc, last);
             gc->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)unreachable);
@@ -1039,7 +1039,7 @@ clear_freelists(void)
     (void)PyContext_ClearFreeList();
 }
 
-// Show stats for objects in each gennerations.
+// Show stats for objects in each generations
 static void
 show_stats_each_generations(GCState *gcstate)
 {
@@ -1058,17 +1058,17 @@ show_stats_each_generations(GCState *gcstate)
         buf, gc_list_size(&gcstate->permanent_generation.head));
 }
 
-/* Deduce wich objects among "base" are unreachable from outside the list
+/* Deduce which objects among "base" are unreachable from outside the list
    and move them to 'unreachable'. The process consist in the following steps:
 
 1. Copy all reference counts to a different field (gc_prev is used to hold
    this copy to save memory).
 2. Traverse all objects in "base" and visit all referred objects using
-   "tp_traverse" and for every visited object, substract 1 to the reference
+   "tp_traverse" and for every visited object, subtract 1 to the reference
    count (the one that we copied in the previous step). After this step, all
    objects that can be reached directly from outside must have strictly positive
    reference count, while all unreachable objects must have a count of exactly 0.
-3. Indentify all unreachable objects (the ones with 0 reference count) and move
+3. Identify all unreachable objects (the ones with 0 reference count) and move
    them to the "unreachable" list. This step also needs to move back to "base" all
    objects that were initially marked as unreachable but are referred transitively
    by the reachable objects (the ones with strictly positive reference count).
@@ -1098,10 +1098,38 @@ deduce_unreachable(PyGC_Head *base, PyGC_Head *unreachable) {
 
     /* Leave everything reachable from outside base in base, and move
      * everything else (in base) to unreachable.
+     *
      * NOTE:  This used to move the reachable objects into a reachable
      * set instead.  But most things usually turn out to be reachable,
-     * so it's more efficient to move the unreachable things.  See note
+     * so it's more efficient to move the unreachable things.  It "sounds slick"
-     ^ [REACHABLE OR UNREACHABLE?] at the file end.
+     * to move the unreachable objects, until you think about it - the reason it
+     * pays isn't actually obvious.
+     *
+     * Suppose we create objects A, B, C in that order.  They appear in the young
+     * generation in the same order.  If B points to A, and C to B, and C is
+     * reachable from outside, then the adjusted refcounts will be 0, 0, and 1
+     * respectively.
+     *
+     * When move_unreachable finds A, A is moved to the unreachable list.  The
+     * same for B when it's first encountered.  Then C is traversed, B is moved
+     * _back_ to the reachable list.  B is eventually traversed, and then A is
+     * moved back to the reachable list.
+     *
+     * So instead of not moving at all, the reachable objects B and A are moved
+     * twice each.  Why is this a win?  A straightforward algorithm to move the
+     * reachable objects instead would move A, B, and C once each.
+     *
+     * The key is that this dance leaves the objects in order C, B, A - it's
+     * reversed from the original order.  On all _subsequent_ scans, none of
+     * them will move.  Since most objects aren't in cycles, this can save an
+     * unbounded number of moves across an unbounded number of later collections.
+     * It can cost more only the first time the chain is scanned.
+     *
+     * Drawback:  move_unreachable is also used to find out what's still trash
+     * after finalizers may resurrect objects.  In _that_ case most unreachable
+     * objects will remain unreachable, so it would be more efficient to move
+     * the reachable objects instead.  But this is a one-time cost, probably not
+     * worth complicating the code to speed just a little.
      */
     gc_list_init(unreachable);
     move_unreachable(base, unreachable);  // gc_prev is pointer again
@@ -1197,7 +1225,7 @@ collect(PyThreadState *tstate, int generation,
         gc_list_merge(young, old);
     }
     else {
-        /* We only untrack dicts in full collections, to avoid quadratic
+        /* We only un-track dicts in full collections, to avoid quadratic
            dict build-up. See issue #14775. */
         untrack_dicts(young);
         gcstate->long_lived_pending = 0;
@@ -2269,39 +2297,3 @@ PyObject_GC_Del(void *op)
     }
     PyObject_FREE(g);
 }
-
-/* ------------------------------------------------------------------------
-Notes
-
-[REACHABLE OR UNREACHABLE?]
-
-It "sounds slick" to move the unreachable objects, until you think about
-it - the reason it pays isn't actually obvious.
-
-Suppose we create objects A, B, C in that order.  They appear in the young
-generation in the same order.  If B points to A, and C to B, and C is
-reachable from outside, then the adjusted refcounts will be 0, 0, and 1
-respectively.
-
-When move_unreachable finds A, A is moved to the unreachable list.  The
-same for B when it's first encountered.  Then C is traversed, B is moved
-_back_ to the reachable list.  B is eventually traversed, and then A is
-moved back to the reachable list.
-
-So instead of not moving at all, the reachable objects B and A are moved
-twice each.  Why is this a win?  A straightforward algorithm to move the
-reachable objects instead would move A, B, and C once each.
-
-The key is that this dance leaves the objects in order C, B, A - it's
-reversed from the original order.  On all _subsequent_ scans, none of
-them will move.  Since most objects aren't in cycles, this can save an
-unbounded number of moves across an unbounded number of later collections.
-It can cost more only the first time the chain is scanned.
-
-Drawback:  move_unreachable is also used to find out what's still trash
-after finalizers may resurrect objects.  In _that_ case most unreachable
-objects will remain unreachable, so it would be more efficient to move
-the reachable objects instead.  But this is a one-time cost, probably not
-worth complicating the code to speed just a little.
------------------------------------------------------------------------- */
-