[docs] [issue28805] Add documentation for METH_FASTCALL and _PyObject_FastCall*()

[STINNER Victor]

STINNER Victor <victor.stinner at gmail.com> added the comment:

> It would also be nice if Cython could use it automatically.

Cython is using FASTCALL since Python 3.6.

----------

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue28805>
_______________________________________