[3pt] Add the new box.ctl.demote() entry

[TarantoolBot]

Add the new box.ctl.demote() entry

Product: Tarantool
Since: 2.10.0-beta1
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_ctl/
https://www.tarantool.io/en/doc/latest/book/replication/repl_sync/
SME: @ sergepetrenko
Peer reviewer: @

Details

box.ctl.demote() is a counterpart to box.ctl.promote() which works
as follows:

• when box.cfg.election_mode is not 'off': make the instance give up
  leadership.
• when box.cfg.election_mode is 'off': write a DEMOTE entry to WAL.

box.ctl.demote() may only be issued on the synchronous transaction
queue owner (i.e. leader when elections are enabled).

A DEMOTE request (DEMOTE = 32) copies PROMOTE behaviour (it clears the
limbo as well), but clears the synchronous transaction queue ownership instead
of assigning it to a new instance.
Requested by @sergepetrenko in tarantool/tarantool@cdb234e.

Definition of done

• Add box.ctl.demote() link to https://www.tarantool.io/en/doc/latest/book/replication/repl_sync/ if necessary