docs: Update FAQ with respect to s3tc support

It's just over 10 months since 17.3.0 was released with s3tc support enabled.
Probably a good idea to update the FAQ page.

v2: Incorporate feedback from Adam Jackson <ajax@redhat.com>

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: 04396a13 ("mesa: Import libtxc_dxtn sources")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit c95dd966)