Mention that effect on options added by the config files.

Author: abidh

flang/docs/FlangDriver.md

@@ -638,4 +638,6 @@ the input argument lists. Edits should be one of the following forms:
   following argument.
 
 - `Ox`: Removes all flags matching `O` or `O[sz0-9]` and adds `Ox` at the end
-  of the command line.
+  of the command line.
+
+This environment variable does not affect the options added by the config files.