update docs to show usage of ipcRenderer.sendTo

docs/api/ipc-renderer.md

@@ -74,6 +74,14 @@ and replies by setting `event.returnValue`.
 **Note:** Sending a synchronous message will block the whole renderer process,
 unless you know what you are doing you should never use it.
 
+### `ipcRenderer.sendTo(windowid, channel, [, arg1][, arg2][, ...])`
+
+* `windowid` Number
+* `channel` String
+* `...args` any[]
+
+Sends a message to a window with `windowid` via `channel`
+
 ### `ipcRenderer.sendToHost(channel[, arg1][, arg2][, ...])`
 
 * `channel` String