[master] add a comment to not edit this xml file.

As briefly mentioned in jabber.

tools/system_messages.py

@@ -58,6 +58,12 @@ SEC_HEADER="""<?xml version="1.0" encoding="UTF-8"?>
 <!ENTITY % version SYSTEM "version.ent">
 %version;
 ]>
+<!--
+     This XML document is generated using the system_messages.py tool
+     based on the .mes message files.
+
+     Do not edit this file.
+-->
 <book>
   <?xml-stylesheet href="bind10-guide.css" type="text/css"?>