[3.8] bpo-37698: Update doc of PyBuffer_ToContiguous (GH-14992) (GH-15999)

https://bugs.python.org/issue37698
(cherry picked from commit 15f5a75)

Co-authored-by: Hai Shi <shihai1992@gmail.com>

Author: matrixise

Doc/c-api/buffer.rst

@@ -491,8 +491,8 @@ Buffer-related functions
 .. c:function:: int PyBuffer_ToContiguous(void *buf, Py_buffer *src, Py_ssize_t len, char order)
 
    Copy *len* bytes from *src* to its contiguous representation in *buf*.
-   *order* can be ``'C'`` or ``'F'`` (for C-style or Fortran-style ordering).
-   ``0`` is returned on success, ``-1`` on error.
+   *order* can be ``'C'`` or ``'F'`` or ``'A'`` (for C-style or Fortran-style
+   ordering or either one). ``0`` is returned on success, ``-1`` on error.
 
    This function fails if *len* != *src->len*.