Include explicit SQL update instructions.

Should have been here in time for 6.2.1 release...

migration/6.2_to_6.2.1

@@ -12,3 +12,11 @@ from psql to update the existing system table:
    where aggname = 'avg' and aggbasetype = 790;
 
 This will need to be done to every existing database, including template1.
+
+Another way to avoid dump/reload is to use the following SQL command
+from psql to update the existing system table:
+
+  update pg_aggregate set aggfinalfn = 'cash_div_flt8'
+   where aggname = 'avg' and aggbasetype = 790;
+
+This will need to be done to every existing database, including template1.