update for 8.4.0-beta2 release

Author: jmckenna

en/MIGRATION_GUIDE.txt

@@ -22,6 +22,8 @@ MapServer 8.2 to 8.4 Migration
 - reminder that since the PROJ 9.1 release, the former `PROJ_LIB` variable has 
   been replaced with `PROJ_DATA`
 
+  .. seealso:: `Setting the location of PROJ files` in :ref:`projection`.
+  
   .. TIP::
      For Windows users, MS4W >=5 leverages the `PROJ_DATA` environment variable, 
      which points to `/ms4w/share/proj/`