[3.13] gh-141004: Document PyBUF_WRITEABLE (GH-141255) (GH-141260)

gh-141004: Document `PyBUF_WRITEABLE` (GH-141255)
(cherry picked from commit 5e5fc04)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Author: miss-islington

Doc/c-api/buffer.rst

@@ -261,6 +261,10 @@ readonly, format
       MUST be consistent for all consumers. For example, :c:expr:`PyBUF_SIMPLE | PyBUF_WRITABLE`
       can be used to request a simple writable buffer.
 
+   .. c:macro:: PyBUF_WRITEABLE
+
+      This is a :term:`soft deprecated` alias to :c:macro:`PyBUF_WRITABLE`.
+
    .. c:macro:: PyBUF_FORMAT
 
       Controls the :c:member:`~Py_buffer.format` field. If set, this field MUST