Binary protocol: describe IPROTO_RAFT_* requests

[TarantoolBot]

<…>=0x0d
IPROTO_BEGIN=0x0e
IPROTO_COMMIT=0x0f
IPROTO_ROLLBACK=0x10
|IPROTO_RAFT_CONFIRM=0x28
IPROTO_RAFT_ROLLBACK=0x29|
IPROTO_PING=0x40
IPROTO_JOIN=0x41
IPROTO_SUBSCRIBE=0x42
IPROTO_<…>

https://www.tarantool.io/en/doc/latest/dev_guide/internals/box_protocol/

Please, mention
IPROTO_RAFT = 0x1e, IPROTO_RAFT_PROMOTE = 0x1f, IPROTO_RAFT_DEMOTE = 0x20 requests.

IPROTO_RAFT request has the following fields:
IPROTO_RAFT_TERM = 0
IPROTO_RAFT_VOTE = 1
IPROTO_RAFT_STATE = 2
IPROTO_RAFT_VCLOCK = 3