Doc: Add M401 and M402 code syntax error and warning messages

Fixes: QTCREATORBUG-28482
Change-Id: I3a8976c0c96c102c9f745a1643d3a02e062658ac
Reviewed-by: Mats Honkamaa <mats.honkamaa@qt.io>
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

doc/qtcreator/src/editors/creator-code-syntax.qdoc

@@ -682,6 +682,20 @@
         \li An import statement has been added twice. For more information,
             see \l{Import Statements}.
 
+    \row
+        \li M401
+        \li Error
+        \li Do not reference the root item as alias
+        \li The alias property cannot reference the root component directly.
+            Create an instance of the component instead.
+
+    \row
+        \li M402
+        \li Warning
+        \li Avoid referencing the root item in a hierarchy
+        \li The alias property cannot reference the root component in a
+            hierarchy. Create an instance of the component instead.
+
     \endtable
 
     \section1 Resetting the Code Model