iproto: document overriding IPROTO request handlers feature

[TarantoolBot]

Product: Tarantool
Since: 2.11

Root document: https://www.tarantool.io/en/doc/latest/dev_guide/internals/box_protocol/
https://www.tarantool.io/en/doc/latest/dev_guide/reference_capi/box/ (box_iproto_override)
https://www.tarantool.io/en/doc/latest/reference/reference_lua/box/ (box.iproto.override)
SME: @ CuriousGeorgiy

Details

For the API description and usage examples, see:

• design document;
• IPROTO request handlers overriding tarantool#7901.
  Requested by @CuriousGeorgiy in tarantool/tarantool@998964c.