Discourage use of the old buffer API funcs

Doc/c-api/objbuffer.rst

 .. highlightlang:: c
 
-.. _abstract-buffer:
+Old buffer API
+--------------
+
+.. deprecated:: 3.0
+
+These functions were part of the "old buffer protocol" API in Python 2.
+In Python 3, these functions are still exposed for ease of porting code.
+They act as a compatibility wrapper around the :ref:`new buffer API
+<bufferobjects>`, but they don't give you control over the lifetime of
+the resources acquired when a buffer is exported.
+
+Therefore, it is recommended that you call :cfunc:`PyObject_GetBuffer`
+(or the ``y*`` or ``w*`` :ref:`format codes <arg-parsing>` with the
+:cfunc:`PyArg_ParseTuple` family of functions) to get a buffer view over
+an object, and :cfunc:`PyBuffer_Release` when the buffer view can be released.
+
 
 Buffer Protocol
 ===============