Fix #3097: Improve documentation of get_action.

Mention that context dicts should not be reused.

ckan/logic/__init__.py

@@ -343,6 +343,12 @@ def get_action(action):
     If a ``context`` of ``None`` is passed to the action function then the
     default context dict will be created.
 
+    .. note::
+
+        Many action functions modify the context dict. It can therefore
+        not be reused for multiple calls of the same or different action
+        functions.
+
     :param action: name of the action function to return,
         eg. ``'package_create'``
     :type action: string