wave: add documentation for undocumented methods

These methods are public but were previously not documented. This caused
the unit tests to fail when I mentioned changes to getparams() in the
whats new ;-)

Author: mbeijen

Doc/library/wave.rst

@@ -277,7 +277,7 @@ Wave_write Objects
    .. method:: getparams()
 
       Return a :func:`~collections.namedtuple`
-      ``(nchannels, sampwidth, framerate, nframes, comptype, compname)``
+      ``(nchannels, sampwidth, framerate, nframes, comptype, compname, format)``
       containing the current output parameters.