docs/userguide: document the --no-startup-id flag

docs/userguide

@@ -1034,6 +1034,35 @@ The criteria +class+, +instance+, +role+, +title+ and +mark+ are actually
 regular expressions (PCRE). See +pcresyntax(3)+ or +perldoc perlre+ for
 information on how to use them.
 
+=== Executing applications (exec)
+
+What good is a window manager if you can’t actually start any applications?
+The exec command starts an application by passing the command you specify to a
+shell. This implies that you can use globbing (wildcards) and programs will be
+searched in your $PATH.
+
+*Syntax*:
+------------------------------
+exec [--no-startup-id] command
+------------------------------
+
+*Example*:
+------------------------------
+# Start the GIMP
+bindsym mod+g exec gimp
+
+# Start the terminal emulator urxvt which is not yet startup-notification-aware
+bindsym mod+enter exec --no-startup-id urxvt
+------------------------------
+
+The +--no-startup-id+ parameter disables startup-notification support for this
+particular exec command. With startup-notification, i3 can make sure that a
+window appears on the workspace on which you used the exec command. Also, it
+will change the X11 cursor to +watch+ (a clock) while the application is
+launching. So, if an application is not startup-notification aware (most GTK
+and Qt using applications seem to be, though), you will end up with a watch
+cursor for 60 seconds.
+
 === Splitting containers
 
 The split command makes the current window a split container. Split containers