Update unix.md - SSH to prompt for installation of X11 on Mac (raspberrypi#582)

* Update unix.md - SSH to prompt for installation of X11 on Mac

Patch to supersede  raspberrypi#398

* copy edit

Author: JamesH65

Diff for: remote-access/ssh/unix.md

@@ -35,20 +35,23 @@ You can also forward your X session over SSH, to allow the use of graphical appl
 ```bash
 ssh -Y [email protected]
 ```
+Note that [X11 is no longer present on Macs with OSX](https://support.apple.com/en-gb/HT201341), so you will have to [download](https://www.xquartz.org/) and install it.
 
-Now you are on the command line as before, but you have the ability to open up graphical windows, for example:
+Now you are on the command line as before, but you have the ability to open up graphical windows. For example, typing:
 
 ```bash
 idle3 &
 ```
 
-This will open up the Python editor IDLE in a graphical window.
+will open up the Python editor IDLE in a graphical window.
+
+Typing:
 
 ```bash
 scratch &
 ```
 
-This will open up Scratch.
+will open up Scratch.
 
 For further documentation on the `ssh` command just enter `man ssh` into the Terminal.