DOC: add documentation for some scalar checks

I went looking for a C API function to check if a value is a scalar or a zero-rank array and found that `PyArray_CheckAnyScalar` is undocumented. This PR adds missing function descriptions.
So it is. Hmm... the scalar references should be consistent in whether scalar is quoted or not, I'd say unquoted. Maybe add `(see ...)` after `python scalar`, where `...` is the first function where python scalar is defined.