man: Add cake_mq documentation to the tc-cake man page

Add documentation for the cake_mq qdisc to the tc-cake man page,
including a section describing the differences between the single-queue
and multi-queue CAKE variants.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>