docs/plugins/: Added wavparse docs.

docs/plugins/: Added wavparse docs.

Original commit message from CVS:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-good-plugins-docs.sgml:
* docs/plugins/gst-plugins-good-plugins-sections.txt:
Added wavparse docs.
* gst/wavparse/gstwavparse.c: (gst_wavparse_class_init),
(gst_wavparse_reset), (gst_wavparse_init),
(gst_wavparse_create_sourcepad), (gst_wavparse_parse_file_header),
(gst_wavparse_stream_init), (gst_wavparse_perform_seek),
(gst_wavparse_stream_headers), (gst_wavparse_send_event),
(gst_wavparse_stream_data), (gst_wavparse_loop),
(gst_wavparse_srcpad_event), (gst_wavparse_sink_activate_pull),
(gst_wavparse_change_state):
* gst/wavparse/gstwavparse.h:
Implement seek in READY (fixes #327658)
Added docs and did some cleanups.