Document new IPROTO_RAFT_PROMOTE request field

[TarantoolBot]

Related dev. issue(s): GitHub link(s)

Product: Tarantool
Since:
Root document:

• https://www.tarantool.io/en/doc/latest/dev_guide/internals/iproto/replication/#iproto-raft-promote
• https://www.tarantool.io/en/doc/latest/dev_guide/internals/iproto/replication/#iproto-raft-demote
• https://www.tarantool.io/en/doc/latest/dev_guide/internals/iproto/keys/#vclock-keys
  SME: @ sergepetrenko

Details

IPROTO_RAFT_PROMOTE and IPROTO_RAFT_DEMOTE requests receive a new key
value pair:

IPROTO_VCLOCK : MP_MAP

The vclock holds a confirmed vclock of the node sending the request.
Requested by @sergepetrenko in tarantool/tarantool@c4415d4.