Map ('5') and allow mod5mask (altgr) as a modifier.

From Artturi Alm (though changed from 'm' to '5')

cwmrc.5

@@ -84,6 +84,8 @@ Meta key.
 Shift key.
 .It Ic 4
 Mod4 (windows) key.
+.It Ic 5
+Mod5 (AltGr) key.
 .El
 .Pp
 The
@@ -101,18 +103,10 @@ The modifier keys come first, followed by a
 .Sq - ,
 then the button number.
 .Pp
-The following modifiers are recognised:
-.Pp
-.Bl -tag -width Ds -offset indent -compact
-.It Ic C
-Control key.
-.It Ic M
-Meta key.
-.It Ic S
-Shift key.
-.It Ic 4
-Mod4 (windows) key.
-.El
+The same modifiers are recognised as for
+.Ar key
+in
+.Nm bind-key .
 .Pp
 The following buttons are recognised:
 .Pp