Document that `IPROTO_EVENT` has sync number

[locker]

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

Product: Tarantool
Since: 2.10.7, 2.11.1, 3.0.0
Audience/target: Connector developers
Root document: IPROTO > Events and subscriptions.
SME: @ locker

Details

Initially the sync number sent by a client in an IPROTO_WATCH request was ignored and IPROTO_EVENT packet didn't have a sync number. There were complaints about it from users so we consider this to be a bug. Now the server sends the same sync number in an IPROTO_EVENT packet as the one sent by the client in the last corresponding IPROTO_WATCH request.