mention that temporary edits in autogenerated pch headers are ok

Change-Id: Ifafa5c8847dec80a3b80afc5e10044f9c1c8d48a

solenv/bin/update_pch.sh

@@ -91,6 +91,13 @@ function list_file_includes()
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
+/*
+ This file has been autogenerated by update_pch.sh . It is possible to edit it
+ manually (such as when an include file has been moved/renamed/removed. All such
+ manual changes will be rewritten by the next run of update_pch.sh (which presumably
+ also fixes all possible problems, so it's usually better to use it).
+*/
+
 EOF
 
     # Library_svx needs this (sendreportw32.cxx)