config: mark instances as non-appointable by the supervised failover

This issue has been created manually due to error in parsing title of https://github.com/tarantool/tarantool-ee/commit/5cbe9e37f067ebc3e20d36eb89391cd913252e95

Dev. issues: (https://github.com/tarantool/tarantool-ee/issues/991, https://github.com/tarantool/tarantool/issues/10842)

Product: Tarantool
Since: 3.3.0
Root document:

• https://www.tarantool.io/en/doc/latest/reference/configuration/configuration_reference/
• (?) https://www.tarantool.io/en/doc/latest/platform/replication/supervised_failover/
  SME: @ georgiy-belyanin

Details

failover.replicasets.<replicaset-name>.learners section allows to
specify instances that are ignored by the supervised failover
coordinator when selecting a master.

Note: if a learner instance is in RW mode, the coordinator stops the
failover process and waits until the instance transitions to RO mode.

Example. Specify "instance-002" and "instance-003" as learners and make
them not selected as masters:

failover:
  replicasets:
    replicaset-001:
      learners:
        - instance-002
        - instance-003

A coordinator configured like this ignores these two instances when
chooses a master and forbids executing "switch" command on them.

Start with the configuration reference and supervised failover documentation pages linked under Root document. Check how the failover.replicasets..learners setting and RW-to-RO behavior are currently described. Done means the documentation explains that learners are excluded from master selection and cannot run the switch command, with the instance-002 and instance-003 YAML example.