docs: fbnic: explain the ring config

fbnic takes 4 parameters to configure the Rx queues. The semantics
are similar to other existing NICs but confusing to newcomers.
Document it.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250626191554.32343-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>