Adds note to templates

This closes #2986, although it was actually closed in Raku/Documentable#17. The problem there is that the variables were not being substituted, simply because no one thought they were actually used. Code to substitute them was added, and that was that.

Author: JJ

template/header.html

@@ -25,5 +25,7 @@
     MENU
   </div>
 </div>
+<!-- please make sure these two CAPITALIZED variables stay there -->
+
 <div id="content" class="pretty-box yellow CONTENT_CLASS">
   EDITURL