Add comments about PyThreadState and the usage of its fields.

Include/pystate.h

@@ -53,12 +53,16 @@ typedef int (*Py_tracefunc)(PyObject *, struct _frame *, int, PyObject *);
 #define PyTrace_C_RETURN 6
 
 typedef struct _ts {
+    /* See Python/ceval.c for comments explaining most fields */
 
     struct _ts *next;
     PyInterpreterState *interp;
 
     struct _frame *frame;
     int recursion_depth;
+    /* 'tracing' keeps track of the execution depth when tracing/profiling.
+       This is to prevent the actual trace/profile code from being recorded in
+       the trace/profile. */
     int tracing;
     int use_tracing;
 
@@ -75,7 +79,7 @@ typedef struct _ts {
     PyObject *exc_value;
     PyObject *exc_traceback;
 
-    PyObject *dict;
+    PyObject *dict;  /* Stores per-thread state */
 
     /* tick_counter is incremented whenever the check_interval ticker
      * reaches zero. The purpose is to give a useful measure of the number