qdoc: Warn if a topic command is not allowed in a \qmlpropertygroup

Change-Id: Icff1f3a4e85ce1eb2afe0a4d66f0728dc414c6ec
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>

src/tools/qdoc/cppcodeparser.cpp

@@ -833,6 +833,10 @@ void CppCodeParser::processQmlProperties(const Doc& doc,
                     }
                 }
             }
+        } else if (qpgn) {
+            doc.startLocation().warning(
+                tr("Invalid use of '\\%1'; not allowed in a '\\%2'").arg(
+                    topic, qmlPropertyGroupTopic.topic));
         }
     }
 }