DOC: filter the correct warning for unique(key, column)

DOC/CLN: clean up indentation and dispatch on numpy version
DOC/BUG: fix overwriting of 'df' variable in doc build
DOC: give deprecation warning about future timeseries broadcasting behavior

DOC/CLN: cleanup
Merge pull request #3815 from cpcloud/doc-build-fix

DOC/BLD: Fix a bunch of doc build warnings that weren't being previously caught