Document backup API enhancements

Product: Tarantool
Since: 3.8

Now box.backup.start() when it is called to backup latest checkpoint
rotates xlog and include it into backup, so backup is done at the moment
of calling this API.

Now box.backup.start() can take a table with options instead of integer
specifying checkpoint number. In this case backup is based on latest
checkpoint and includes rotated xlog. Supported options are:

• from_vclock, if it is specified the backup will be incremental, i.e. it
  will only include xlogs since given vclock. The vclock should point to
  the end of the previous full or incremental backup (vclock field
  of box.backup.info()).
• ttl sets expiration time for a backup. After this time the backup
  will be stopped. If not set then default expiration time will be used
  (see compat.box_backup_default_ttl).

New box_backup_default_ttl compat option controls default expiration
time for a backup. It is infinite when compat is set to old (versions
3.x default) and 1 hour when it is set to new (versions 4.x default).

box.backup.info() is intoroduced to query details about backup in
progress. It returns nil if there is such. Example output:

tarantool> box.backup.info()
---
- type: full
  files:
  - ./00000000000000000004.snap
  vclock: {1: 4}
  expires_at: 1781082529.6392
...

It's fields are:

• files - backup files, same list as box.backup.start() returns.
• type - 'incremental' or 'full'.
• vclock - vclock of the last statement in the backup (incremental
  backup can be empty, in this case it is from_vclock specified on
  start).
• prev_vclock - vclock of previous backup (from_vclock specified on
  start) for incremental backups, nil for full backups.
• expires_at - Unix-time when backup will be expired. nil if
  expiration time is infinite.
  Requested by @nshy in https://github.com/tarantool/tarantool/commit/b39d837caafa01e9fad93c99e8178983dc421659.

Search the Tarantool documentation for the existing box.backup.start() API reference and its surrounding backup material. Document checkpoint rotation, incremental and TTL options, compat.box_backup_default_ttl, and box.backup.info(), including its fields and example output. Done when the documented behavior matches the issue and the new API details are easy to find.