Partial Fix gh-869 Сonfusing wording in box.backup page

Fixed description regarding garbage collection in backup.start()

Author: Vladimir I

doc/book/admin/backups.rst

@@ -88,15 +88,14 @@ checkpoint.
 To take a mixed backup:
 
 1. Issue :ref:`box.backup.start() <admin-backups-backup_start>` on the
-   :ref:`administrative console <admin-security>`. This will suspend
-   garbage collection till the next ``box.backup.stop()`` and
-   will return a list of files to back up.
+   :ref:`administrative console <admin-security>`. This will return a list of
+   files to back up and suspend garbage collection of them till the next ``box.backup.stop()``.
 
 2. Copy the files from the list to a safe location. This will include memtx
    snapshot files, vinyl run and index files, at a state consistent with the
    last checkpoint.
 
-3. Issue ``box.backup.stop()`` so the garbage collector can continue.
+3. Issue ``box.backup.stop()`` so the garbage collector can continue as usual.
 
 .. _admin-backups-cont_remote_backup_memtx: