docs: do not document required minimum

We already document that shaderClipDistance is required, and for that
feature to be supported maxClipDistances needs to be at least 8. So
there's no point in documenting the maxClipDistances-requirement.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7286>