Issue #4129: More documentation pointers about int -> Py_ssize_t.

Also fix up the documentation for PyObject_GC_Resize(). It seems that since
it first got documented, the documentation was actually for
_PyObject_GC_Resize().

Doc/c-api/gcsupport.rst

@@ -48,12 +48,20 @@ Constructors for container types must conform to two rules:
    Analogous to :cfunc:`PyObject_NewVar` but for container objects with the
    :const:`Py_TPFLAGS_HAVE_GC` flag set.
 
+   .. versionchanged:: 2.5
+      This function used an :ctype:`int` type for *size*. This might require
+      changes in your code for properly supporting 64-bit systems.
 
-.. cfunction:: PyVarObject * PyObject_GC_Resize(PyVarObject *op, Py_ssize_t)
+
+.. cfunction:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)
 
    Resize an object allocated by :cfunc:`PyObject_NewVar`.  Returns the
    resized object or *NULL* on failure.
 
+   .. versionchanged:: 2.5
+      This function used an :ctype:`int` type for *newsize*. This might
+      require changes in your code for properly supporting 64-bit systems.
+
 
 .. cfunction:: void PyObject_GC_Track(PyObject *op)