config: `bootstrap_strategy` now grants privileges in some cases

[TarantoolBot]

replication.bootstrap_strategy needs a note on the granted privileges
at the end of the various bootstrap strategies descriptions.

Note: when using bootstrap strategies supervised or native with a
supervised failover (see replication.failover configuration option)
Tarantool automatically grants the guest user privileges allowing to
execute the internal failover.execute call for performing the initial
cluster bootstrap.
Requested by @georgiy-belyanin in tarantool/tarantool@e57fc4c.