Add documentation for id3demux.

Add documentation for id3demux.

Original commit message from CVS:
2005-12-21  Jan Schmidt  <thaytan@mad.scientist.com>

* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-good-plugins-docs.sgml:
* docs/plugins/gst-plugins-good-plugins-sections.txt:
* docs/plugins/gst-plugins-good-plugins.args:
* gst/id3demux/gstid3demux.c: (gst_id3demux_get_type),
(gst_id3demux_base_init), (gst_id3demux_class_init),
(gst_id3demux_chain):
* gst/id3demux/gstid3demux.h:
Add documentation for id3demux.
Don't fail if the first buffer is not at offset 0, just
attempt to typefind and do pass through
Rename the gst_type function from gst_gst_id3demux..