mesa: Add comments about bit-ordering of new XRGB/XBGR formats.

mesa: Add comments about bit-ordering of new XRGB/XBGR formats.

Marek added these new formats in commit f9fa725690c470daf308, but
without comments relating to the packing.  Sometimes the naming is
confusing, so these comments are helpful in determining whether two
formats are compatible.

The new comments are based on my reading of format_unpack.c.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>