Document export of current session's identifier to C API

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#7895

Product: Tarantool
Since: 2.11
Root document: https://www.tarantool.io/en/doc/latest/dev_guide/reference_capi/box/
SME: @ CuriousGeorgiy

Details

For the API description and usage examples, see the design document.

Requested by @ CuriousGeorgiy in tarantool/tarantool@9b0a767.