As Serhiy noted, argument parsing API (_PyArg_ParseStack) is not public too.
So METH_FASTCALL is incomplete for pure C extension authors even if it's documented.
So I don't have strong opinion for documenting it on 3.7.
Consensus about not changing it (without METH_KEYWORDS) on 3.8 seems enough
to me (and Cython).
Then, _PyArg_ParseStack API should be considered first for make it public on Python 3.8.
(bikeshedding: The name *Stack* feels not good. It implies Python VM stack. But this
API can be used not only with VM stack.)
METH_KEYWORDS first. Making _PyObject_FastCall() public is significant step for 3.8.