gallium/docs: many updates to screen.rst

gallium/docs: many updates to screen.rst

Put PIPE_BIND_ and PIPE_TEXTURE_GEOM_ prefixes on token names so
that they can be found with grep.  This needs to be done in more places.

Corrected/improved a lot of information and grammer.

I don't know how to properly format everything - someone else can take
care of that.