Add initial documentation for the MXF plugin and mxfdemux.

Add initial documentation for the MXF plugin and mxfdemux.

Original commit message from CVS:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-bad-plugins-docs.sgml:
* docs/plugins/gst-plugins-bad-plugins-sections.txt:
* docs/plugins/gst-plugins-bad-plugins.args:
* docs/plugins/gst-plugins-bad-plugins.hierarchy:
* docs/plugins/gst-plugins-bad-plugins.interfaces:
* docs/plugins/gst-plugins-bad-plugins.prerequisites:
* docs/plugins/inspect/plugin-ladspa.xml:
* docs/plugins/inspect/plugin-musepack.xml:
* docs/plugins/inspect/plugin-mxf.xml:
* gst/mxf/mxfdemux.c:
* gst/mxf/mxfdemux.h:
Add initial documentation for the MXF plugin and mxfdemux.