docs/gallium: Note samplers are not used for txf

docs/gallium: Note samplers are not used for txf

Now that PIPE_CAP_TEXTURE_BUFFER_SAMPLER is gone, txf does not require samplers
for any texture on any Gallium driver. NIR already requires drivers to ignore
sampler_index for non-sampler operation (mainly txf), and nowadays all Gallium
drivers ingest NIR. So, document that samplers aren't bound for txf (etc) as
part of the Gallium frontend-driver contract.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Suggested-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22223>