Flags documentation: -fwarn-unrecognized-pragmas is enabled by -Wall (#6050)

docs/users_guide/using.xml

@@ -988,7 +988,6 @@ test.hs:(5,4)-(6,7):
             <option>-fwarn-incomplete-uni-patterns</option>,
             <option>-fwarn-incomplete-record-updates</option>,
             <option>-fwarn-monomorphism-restriction</option>,
-            <option>-fwarn-unrecognised-pragmas</option>,
             <option>-fwarn-auto-orphans</option>,
             <option>-fwarn-implicit-prelude</option>.</para>
         </listitem>