DOC: make api docs reflect api usage (#4539)

Closes #4539.

This removes all use of `.. currentmodule:: pandas.some.submodule` (in favor of `.. currentmodule:: pandas`) in api.rst. Rationale is that docs should reflect the user api usage.

Only exception are `pandas.io.sql.read_frame/write_frame` and `pandas.io.stata.StataReader` methods, because they are not available in the top-level pandas namespace.

All old links will remain valid because they are still generated because I added all those in a comment at the bottom in api.rst (only not for Index, because this was only added very recently in api.rst).  
The **downside** with this approach is that you now get warnings when building the docs (`document isn't included in any toctree` for these functions). I am not sure how I can prevent this while keeping the old links but not including it visually in api.rst.