docs/plugins/Makefile.am: also check .cc files for gtk-doc markup

docs/plugins/Makefile.am: also check .cc files for gtk-doc markup

Original commit message from CVS:

* docs/plugins/Makefile.am:
also check .cc files for gtk-doc markup
* configure.ac:
* docs/plugins/gst-plugins-good-plugins-docs.sgml:
* docs/plugins/gst-plugins-good-plugins-sections.txt:
* tests/check/Makefile.am:
* tests/check/elements/id3v2mux.c: (id3v2mux_suite), (main):
* ext/Makefile.am:
* ext/taglib/Makefile.am:
* ext/taglib/gstid3v2mux.h:
* ext/taglib/gsttaglibmux.c:
* ext/taglib/gsttaglibmux.h:
move taglib-based id3v2muxer to -good.  Fixes #336110.