More Docs updates.

More Docs updates.

Original commit message from CVS:
More Docs updates.
Added plugin documentation. I fear we need a gstdoc implementation
that loads plugins and does introspection on them. I think we should
automatically create the docs for the pads and mime types the plugins
provide. Does anyone have enough perl knowledge to add these features? I
allready changed the C code to output the pad definitions but my perl
knowledge is too limited, for now, to implement the rest of the needed
functionality...