bpo-42528: Improve the docs of most Py*_Check{,Exact} API calls (GH-23602) (GH-24139)

bpo-42528: Improve the docs of most Py*_Check{,Exact} API calls (GH-23602) (GH-24139)

I think that none of these API calls can fail, but only few of them are
documented as such. Add the sentence "This function always succeeds" (which is
the same already used e.g. by PyNumber_Check) to all of them.
(cherry picked from commit 315fc52db17b19fe30aa9193f26adf69e18d8844)

Co-authored-by: Antonio Cuni <anto.cuni@gmail.com>
Co-authored-by: Antonio Cuni <anto.cuni@gmail.com>