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...