DOC: update docstring validation script
Follow-up on https://github.com/pandas-dev/pandas/pull/19898

cc @datapythonista 
Something related, but the `Series.str`, `Series.cat`, `Series.dt` accessors themselves are not included in the overview (spreadsheet) I think. They should be documented as well, so that's something to fix (or add manually) The csv generated should have a row for each item in `doc/source/api.rst`. Unless there is a bug in the script, if they are missing it's because they are missing from that document, and they should be added there if we want to include them in the documentation. They are in api.rst, but as a comment, so depending on how it is parsed that might give a problem: https://github.com/pandas-dev/pandas/blame/master/doc/source/api.rst#L674-L681