Document _space_sequence and _vspace_sequence system spaces

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#7858

Product: Tarantool
Since: 2.11
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/system_views/
SME: @ locker

Details

The _space_sequence system space was added long time ago (in 1.7.5)
along with the _sequence and _sequence_data system spaces, but it
was never documented. The space is used to attach sequences to spaces
and has the following fields:

1. 'id', type 'unsigned'. Space id.
2. 'sequence_id', type 'unsigned'. Id of the attached sequence.
3. 'is_generated', type 'boolean'. True if the sequence was created
   automatically (space:create_index('pk', {sequence = true}))
4. 'field', type 'unsigned'. Id of the space field that is set using
   the attached sequence.
5. 'path', type 'string'. Path to the data within the field that is set
   using the attached sequence.

The _vspace_sequence is a system space view of the _space_sequence
space that, like any other system space view, shows only rows accessible
by the current user. It will be introduced in Tarantool 2.10.5.
Requested by @ locker in tarantool/tarantool@06ee113.