net: bridge: add document for IFLA_BR enum

Add document for IFLA_BR enum so we can use it in
Documentation/networking/bridge.rst.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>