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

[jorisvandenbossche]

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.

[jorisvandenbossche]

OK, solved the downside by adding a hidden toctree. So now all old links remain valid, and no warnings are generated. The only downside is that there is a big chunk of lines at the bottom in api.rst that is maybe somewhat noisy, just to keep the links.

[jtratner]

If you can add comments, can you leave a note saying that its' there to maintain links?

[jorisvandenbossche]

I did, quoting from the commit

HACK - see github issue #4539. To ensure old links remain valid, include
here the autosummaries with previous currentmodules as a comment and add 
them to a hidden toctree (to avoid warnings):

If it is not clear enough, please suggest something.

Travis had a failing (related to sql: test_basic (pandas.io.tests.test_sql.TestMySQL). Strange, because it is not related to the docs, and before I updated the commit (a forced push), the commit was green. Is this a problem?

[jtratner]

sorry missed this

[jtratner]

yep unrelated