DOC: add back currentmodule to api.rst
xref https://github.com/pandas-dev/pandas/pull/24191

@datapythonista I don't understand why, but the `.. currentmodule:: pandas` does not seem to be picked up from `header.rst`, giving extra warnings and missing links (because in api.rst, everything up to the first inline `currentmodule`, is not found)

See eg http://pandas-docs.github.io/pandas-docs-travis/api.html, only from json there are actual links.