[routing-manager] update legacy comments in routing manager (#7533)
[routing-manager] update legacy comments in routing manager (#7533)

This commit updates some legacy comments when evaluating routing
policy, especially about the legacy Router Advertisement timer.
abtink/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,abtink/openthread,srickardti/openthread,srickardti/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,openthread/openthread