Fix gh-869 Сonfusing wording in box.backup page

Changed description as intended. Consider review.

Author: Vladimir I

doc/1.10/book/admin/backups.rst

@@ -22,20 +22,9 @@ backup.start() and backup.stop()
 Two functions are helpful for backups in certain situations.
 
 ``box.backup.start()`` informs the server that some activities
-that might interfer with backup should be suspended -- suspend checkpointing,
-suspend Tarantool garbage collection, and effectively enter read-only mode.
-
-Later ``box.backup.stop()`` informs the server that
-normal operations may resume. Starting with Tarantool 1.10.1 there is a new
-optional argument, ``box.backup.start(n)``, where ``n`` indicates the checkpoint
-to use relative to the latest checkpoint -- for example ``n = 0`` means
-"backup will be based on the latest checkpoint", ``n = 1`` means "backup will
-be based on the first checkpoint before the latest checkpoint
-(counting backwards)", and so on,
-and the default value for ``n`` is zero.
-
-``box.backup.start()`` returns a table with the names of snapshot
-and vinyl files that should be copied. Example:
+that might interfere with backup should be suspended (suspend checkpointing,
+Tarantool garbage collection etc) and returns a table with the names of
+snapshot and vinyl files that should be copied. Example:
 
 .. code-block:: tarantoolsession
 
@@ -47,6 +36,20 @@ and vinyl files that should be copied. Example:
       - ./513/0/00000000000000000002.run
     ...
 
+.. NOTE::
+
+    To guarantee an opportunity to copy this files Tarantool will not delete them.
+    But there will be no read-only mode nor checkpoints postponed in this state.
+
+Later ``box.backup.stop()`` informs the server that
+normal operations may resume. Starting with Tarantool 1.10.1 there is a new
+optional argument, ``box.backup.start(n)``, where ``n`` indicates the checkpoint
+to use relative to the latest checkpoint -- for example ``n = 0`` means
+"backup will be based on the latest checkpoint", ``n = 1`` means "backup will
+be based on the first checkpoint before the latest checkpoint
+(counting backwards)", and so on,
+and the default value for ``n`` is zero.
+
 .. _admin-backups-hot_backup_memtx:
 
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