Amend docs with immutable property attribute.

Include warning about updates not being supported for a propety
based on its `immutable` value being True.

Change-Id: I10fe495a2a35429b3d6e254c23ae91732e2e4dd8
Implements: blueprint implement-aws-updates-not-supported

doc/source/ext/resources.py

@@ -27,7 +27,6 @@ from heat.engine import support
 from heat.openstack.common.gettextutils import _
 
 
-
 global_env = environment.Environment({}, user_env=False)
 
 
@@ -219,6 +218,11 @@ Resources:
             para = nodes.paragraph('',
                                    _('Can be updated without replacement.'))
             definition.append(para)
+        elif prop.immutable:
+            para = nodes.paragraph('', _('Updates are not supported. '
+                                         'Resource update will fail on any '
+                                         'attempt to update this property.'))
+            definition.append(para)
         else:
             para = nodes.paragraph('', _('Updates cause replacement.'))
             definition.append(para)