Add rough reference documentation for the main encoding calls.

Add rough reference documentation for the main encoding calls.

These are a bit rough in that the main calls are declared in codec.h,
and not vorbisenc.h as one might expect for the encoder. Nevertheless
I felt this was helpful to those trying to use the encoder since there
is no 'Programming with libvorbis' documenation at all.

The error return values listed don't entirely match what the code will
actually provide, but they're all good ones to check for anyway.

Note of the new pages have hyperlinked function or structure names.

svn path=/trunk/vorbis/; revision=16756