[2pt] Document new binary protocol key and subscribe request changes

[Totktonada]

Product: Tarantool
Since: 2.10.0-beta2
Audience/target: developers
Root document: https://www.tarantool.io/en/doc/latest/dev_guide/internals/box_protocol/
SME: @ sergepetrenko

Details

Add key IPROTO_ID_FILTER = 0x51 to the internals reference.
This is an optional key used in SUBSCRIBE request followed by an array
of ids of instances whose rows won't be relayed to the replica.

SUBSCRIBE request is supplemented with an optional field of the
following structure:

+====================+
|      ID_FILTER     |
|   0x51 : ID LIST   |
| MP_INT : MP_ARRRAY |
|                    |
+====================+

The field is encoded only when the id list is not empty.

Requested by @sergepetrenko in tarantool/tarantool@45de990