Added docs.

Added docs.

Original commit message from CVS:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-base-plugins-docs.sgml:
* docs/plugins/gst-plugins-base-plugins-sections.txt:
* ext/ogg/gstoggdemux.c: (gst_ogg_page_copy), (gst_ogg_page_free),
(gst_ogg_pad_query_types), (gst_ogg_pad_submit_page),
(gst_ogg_chain_reset), (gst_ogg_chain_new_stream),
(gst_ogg_demux_perform_seek):
* ext/ogg/gstoggdemux.h:
Added docs.
Add some more comments.
Small cleanups.