`election_mode` update

Here: https://www.tarantool.io/en/doc/latest/reference/configuration/#confval-election_mode.

The doc says that the election_mode='off' makes it not
participate in the election process, and it won't run the Raft
state machine.

But it is gonna change. Need to update the text between
"Possible values" and "Since version 2.8.2, .....".

Like this:

Participation of a replica set node in the automated leader
election can be configured with this option.

The default value is `'off'`.

All nodes run the Raft state machine internally, talking to other
nodes according to the Raft leader election protocol. All nodes
can vote for other nodes in elections and ack synchronous
transactions except for anonymous replicas (they are basically
ignored by elections and synchronous transactions ack counting).

When the mode is `'off'`, the node can be writable even when it is
not the leader. Normally in this state the Raft state machine is
just idle on all the nodes, and the leadership management is done
externally (if done at all).

If in `'off'` mode the node sees the synchronous transaction queue
claimed by any node (`box.info.synchro.queue.owner` is not 0),
then it will be read-only unless it is the owner of the queue and
is the elected leader.

To disable synchronous replication entirely and to be able to make
nodes writable regardless of Raft and the synchronous transaction
queue you can switch into `'off'` mode, and then do
`box.ctl.promote()` + `box.ctl.demote()`.

The mode `'off'` is mostly just a way to run asynchronous
replication, and also a way to get off the synchronous
replication. It must not be used for actual production workload as
it can result into split-brain errors even in a seemingly healthy
replicaset.

The usage ways are normally either mode `'off'` on all nodes and
no `box.ctl.promote()` at all, or election modes `'voter'`,
`'candidate'` and `'manual'` on all nodes and full reliance on the
synchronous replication and Raft elections.

When a node is a `'voter'`, it will be able to vote, but will not
be a leader. It will also be read-only guaranteed.

When a node is a `'candidate'`, it will watch the state of the
current leader, and will actively try to get itself elected when
the leader is not healthy.

Requested by @Gerold103 in https://github.com/tarantool/tarantool/commit/a315dd70c20ea36b80b1ab3ce73d8eee66dffa89.

Open the configuration reference at the linked Tarantool documentation page and locate the election_mode section. Replace the text between “Possible values” and “Since version 2.8.2” with the wording provided in the issue, then verify the rendered section accurately reflects the updated election behavior.