Document public role rw access on _session_settings space

[TarantoolBot]

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/_user/#box-space-user
https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/system_views/
https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/_session_settings/

Public role (which is granted to new users by default) now has read,
write access on _session_settings space.

To-do:

• document public role rw access
• check formatting in root docs

Requested by @CuriousGeorgiy in tarantool/tarantool@18eae24.