document that -fglasgow-exts is needed for RULES to work

diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index 319104e..3801f96 100644 (file)
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -5378,7 +5378,9 @@ The programmer can specify rewrite rules as part of the source program
 (in a pragma).  GHC applies these rewrite rules wherever it can, provided (a) 
 the <option>-O</option> flag (<xref linkend="options-optimise"/>) is on, 
 and (b) the <option>-frules-off</option> flag
-(<xref linkend="options-f"/>) is not specified.
+(<xref linkend="options-f"/>) is not specified, and (c) the
+<option>-fglasgow-exts</option> (<xref linkend="options-language"/>)
+flag is active.
 </para>
 
 <para>