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/socketmodule.c

@@ -1402,8 +1402,8 @@ PySocketSock_recv(PySocketSockObject *s, PyObject *args)
 		Py_DECREF(buf);
 		return s->errorhandler();
 	}
-	if (n != len && _PyString_Resize(&buf, n) < 0)
-		return NULL;
+	if (n != len)
+		_PyString_Resize(&buf, n);
 	return buf;
 }