Missing docs for rdedfann (and other functions)

[bemoody]

The function wfdb.rdedfann (alias wfdb.io.rdedfann) is not listed in the public documentation.

Make a list here of other functions that should be considered part of the public API, but are not listed in docs/wfdb.rst / docs/io.rst / docs/plot.rst / docs/processing.rst.