docs/plugins/: Fix hierarchy, added some more elements to the docs.

docs/plugins/: Fix hierarchy, added some more elements to the docs.

Original commit message from CVS:
* docs/plugins/gst-plugins-base-plugins-docs.sgml:
* docs/plugins/gst-plugins-base-plugins.args:
* docs/plugins/gst-plugins-base-plugins.hierarchy:
* docs/plugins/gst-plugins-base-plugins.interfaces:
* docs/plugins/gst-plugins-base-plugins.signals:
Fix hierarchy, added some more elements to the docs.
* gst/ffmpegcolorspace/gstffmpegcolorspace.c:
(gst_ffmpegcsp_get_type):
* gst/ffmpegcolorspace/gstffmpegcolorspace.h:
Fix docs for ffmpegcolorspace.