Document failover_interval router option

[TarantoolBot]

The interval in seconds between actions by the failover service.
For replicas it means, how often pings are sent, for replicasets -
how often the prioritized replica is checked for relevance.

The option is specified in the root of the config. Default value
is 1.
Requested by @Serpentian in tarantool/vshard@3275466.