DOC: complete signal functions listed in reference guide.

Also edit some docstrings, and add a missing __all__ dict.
DOC: update fftpack entries in refguide.
MAINT: add tools/refguide_check.py, which helps check for missing objects in docs.
Merge pull request #3693 from rgommers/signal-docs

DOC: update refguide entries for signal and fftpack