gallium/docs: improve text about resources a bit.

gallium/docs: improve text about resources a bit.

This clarifies some things and gets rid of some old stuff.
The most significant one is probably that buffers cannot have formats
(nearly all drivers completely ignored format and used width0 as byte size
already in any case). There seems to be no use case for "structured" buffers.
(Note while d3d11 has new Structured Buffers, these still aren't associated
with a format, rather a byte stride, which we can't do yet either way.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>