fix gh-1166 Harmonize replication bootstrap config

Author: lenkis

doc/book/replication/repl_bootstrap.rst

@@ -322,9 +322,14 @@ URIs of both masters in the replica set and say
 ``print('box.once executed on master #1')`` so it will be clear when and where the
 ``box.once()`` logic is executed.
 
-Now we can launch the two masters. Again, the launch order doesn't matter.
-The ``box.once()`` logic will also be executed only once, at the master which
-is elected as the replica set :ref:`leader <replication-leader>` at bootstrap.
+Now we can launch the two masters.
+
+.. NOTE::
+
+    Tarantool guarantees that ``box.once()`` logic will be executed once only
+    for a single instance. Starting a master-master replica set in parallel
+    can cause repeated execution of ``box.once``. That, particularly, can result
+    in data inconsistency.
 
 .. code-block:: console