Rewrote Pokémon form group descriptions.

They now use our modified reST to link a few things like "Gracidea",
mention HG/SS where applicable, and are much more correct in general.

I might have missed some odd thing, and there are still a couple of
stylistic issues.  Rotom's description is really long, for example, and
I'm not sure what to do about that; all of it seems fairly important.