Document GADTSyntax extension

docs/users_guide/flags.xml

@@ -762,6 +762,13 @@
 	      <entry>dynamic</entry>
 	      <entry><option>-XNoGADTs</option></entry>
 	    </row>
+	    <row>
+	      <entry><option>-XGADTSyntax</option></entry>
+	      <entry>Enable <link linkend="gadt-style">generalised algebraic data type syntax</link>.
+	      </entry>
+	      <entry>dynamic</entry>
+	      <entry><option>-XNoGADTSyntax</option></entry>
+	    </row>
 	    <row>
 	      <entry><option>-XTypeFamilies</option></entry>
 	      <entry>Enable <link linkend="type-families">type families</link>.</entry>

docs/users_guide/glasgow_exts.xml

@@ -2434,7 +2434,8 @@ declarations.  Define your own instances!
 <sect2 id="gadt-style">
 <title>Declaring data types with explicit constructor signatures</title>
 
-<para>GHC allows you to declare an algebraic data type by 
+<para>When the <literal>GADTSyntax</literal> extension is enabled,
+GHC allows you to declare an algebraic data type by
 giving the type signatures of constructors explicitly.  For example:
 <programlisting>
   data Maybe a where