bridge: make mcast_flood description consistent

bridge: make mcast_flood description consistent

This patch simply changes the description of the mcast_flood flag
with "flood" instead of "be flooded with" to avoid confusion, and be
consistent with the description of the flooding flag, which "Controls
whether a given port will *flood* unicast traffic for which there is
no FDB entry."

At the same time, fix the documentation for the "flood" flag which
is incorrectly described as "flooding on" or "flooding off".

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>