Repair widespread misuse of _PyString_Resize. Since it's clear people

don't understand how this function works, also beefed up the docs.  The
most common usage error is of this form (often spread out across gotos):

	if (_PyString_Resize(&s, n) < 0) {
		Py_DECREF(s);
		s = NULL;
		goto outtahere;
	}

The error is that if _PyString_Resize runs out of memory, it automatically
decrefs the input string object s (which also deallocates it, since its
refcount must be 1 upon entry), and sets s to NULL.  So if the "if"
branch ever triggers, it's an error to call Py_DECREF(s):  s is already
NULL!  A correct way to write the above is the simpler (and intended)

	if (_PyString_Resize(&s, n) < 0)
		goto outtahere;

Bugfix candidate.

Objects/fileobject.c

@@ -1317,9 +1317,7 @@ file_readlines(PyFileObject *f, PyObject *args)
 			goto error;
 	}
   cleanup:
-	if (big_buffer) {
-		Py_DECREF(big_buffer);
-	}
+	Py_XDECREF(big_buffer);
 	return list;
 }