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.

Modules/cPickle.c

@@ -1252,14 +1252,8 @@ modified_EncodeRawUnicodeEscape(const Py_UNICODE *s, int size)
 			*p++ = (char) ch;
 	}
 	*p = '\0';
-	if (_PyString_Resize(&repr, p - q))
-		goto onError;
-
+	_PyString_Resize(&repr, p - q);
 	return repr;
-
-  onError:
-	Py_DECREF(repr);
-	return NULL;
 }