vector/cmatch: Deprecate @const@ on shorthand templates

* vector/cmatch: Display warning on use of @const@ with short-hand
  comparison templates.

core/vector/cmatch.xml

@@ -192,6 +192,12 @@
       <param name="@const@" desc="Match against constant value" />
       <param name="@value@" desc="Match against variable" />
 
+      <if name="@const@">
+        <warning>
+          @const@ is deprecated; use @value@ with a #-prefix instead.
+        </warning>
+      </if>
+
       <match on="@on@">
         <dyn-node name="c:{@cmp@}">
           <if name="@const@">