Add doc note for glance-api container

Due this change [1] marked doc impact for start
only one glance-api when file backed is used, this
change adds a note in multinode guide to notify users
about this behaviour.

[1] https://review.openstack.org/#/c/448654/
Closes-Bug: #1706901

Change-Id: I7d5a3bea00d3f0ee990b0f23acea4d269a92b377

doc/multinode.rst

@@ -188,6 +188,12 @@ Deploying Kolla
     ``keepalived_virtual_router_id``. The ``keepalived_virtual_router_id`` should
     be unique and belong to the range 0 to 255.
 
+.. note::
+
+   If glance is configured to use ``file`` as backend, only one ``glance_api``
+   container will be started. ``File`` is enabled by default when no other
+   backend is specified in globals.yml
+
 First, check that the deployment targets are in a state where Kolla may deploy
 to them: