introduce the sync IPC command

Sending the sync command via IPC ensures pending IPC messages are handled by i3
before the sync response is read. This is rarely useful for direct IPC
connections to i3, but becomes useful when synchronizing with i3bar, which might
have pending IPC messages in response to button clicks.

docs/ipc

@@ -65,6 +65,7 @@ to do that).
 | 8 | +GET_BINDING_MODES+ | <<_binding_modes_reply,BINDING_MODES>> | Gets the names of all currently configured binding modes.
 | 9 | +GET_CONFIG+ | <<_config_reply,CONFIG>> | Returns the last loaded i3 config.
 | 10 | +SEND_TICK+ | <<_tick_reply,TICK>> | Sends a tick event with the specified payload.
+| 11 | +SYNC+ | <<_sync_reply,SYNC>> | Sends an i3 sync event with the specified random value to the specified window.
 |======================================================
 
 So, a typical message could look like this:
@@ -654,6 +655,18 @@ events generated prior to the +SEND_TICK+ message (happened-before relation).
 { "success": true }
 -------------------
 
+[[_sync_reply]]
+=== SYNC reply
+
+The reply is a map containing the "success" member. After the reply was
+received, the https://i3wm.org/docs/testsuite.html#i3_sync[i3 sync message] was
+responded to.
+
+*Example:*
+-------------------
+{ "success": true }
+-------------------
+
 == Events
 
 [[events]]