Document running multiple engines at once

doc/user/engine.rst

@@ -15,9 +15,17 @@ Python interpreter.
 Namespace
 ---------
 
+Namespaces allow to identify and isolate multiple Spinach engines running on
+the same Python interpreter and/or sharing the same Redis server.
+
+Having multiple engines on the same interpreter is rare but can happen when
+using the Flask integration with an app factory. In this case using different
+namespaces is important to avoid signals sent from one engine to be received by
+another engine.
+
 When multiple Spinach Engines use the same Redis server, for example when
-production and staging share the same database, different namespaces are used
-to make sure they do not step on each other's feet.
+production and staging share the same database, different namespaces must be
+used to make sure they do not step on each other's feet.
 
 The production application would contain::