doc: document time units for Linux bridge timing options

Specifying that forward_delay, hello_time, max_age and mac_ageing_time
  are in seconds, and multicast timing intervals are in centiseconds
  (1/100 of a second).

  Resolving: https://github.com/nmstate/nmstate/issues/1216

Signed-off-by: Martina Sekerakova <msekerak@redhat.com>