Add documentation for urgent command

sway/sway.5.scd

@@ -499,6 +499,11 @@ config after the others, or it will be matched instead of the others.
 	*unmark* will remove _identifier_ from the list of current marks on a
 	window. If _identifier_ is omitted, all marks are removed.
 
+*urgent* enable|disable|allow|deny
+	Using _enable_ or _disable_ manually sets or unsets the window's urgent
+	state. Using _allow_ or _deny_ controls the window's ability to set itself
+	as urgent. By default, windows are allowed to set their own urgency.
+
 *workspace* [number] <name>
 	Switches to the specified workspace. The string "number" is optional and is
 	used to sort workspaces.