Document a way to check `backup` status

Product: Tarantool
Audience/target: admin
Root document: https://www.tarantool.io/en/doc/latest/book/admin/backups/ ?
SME: @ alyapunov

This is an example on how to check if Tarantool backup mode is enabled:

unix/:./tmp/run/router.control> box.backup.start()
---
- - /home/user/.../tmp/data/router/00000000000000000000.snap
...

unix/:./tmp/run/.router.control> box.info.gc()
---
- signature: 0
  checkpoint_is_in_progress: false
  consumers: []
  is_paused: false
  vclock: {}
  checkpoints:
  - references:
    - backup
    vclock: {}
    signature: 0
...

unix/:./tmp/run/router.control> box.backup.stop()
---
...

unix/:./tmp/run/router.control> box.info.gc()
---
- signature: 0
  checkpoint_is_in_progress: false
  consumers: []
  is_paused: false
  vclock: {}
  checkpoints:
  - references: []
    vclock: {}
    signature: 0
...

unix/:./tmp/run/router.control>

courtesy of @no1seman

Oneliner according to console output above:

box.info.gc().checkpoints[1].references[1] == 'backup'

This is a bit uncomfortable way, but we have what we have. Can you check if it is the only way? If so -- document it?

Start with the Tarantool backups documentation at the linked root document and review the shown box.backup.start(), box.backup.stop(), and box.info.gc() output. Check whether another way to determine backup status exists, then document the confirmed approach and its result in that section.