Update yoyo function in user guide

The yoyo function currently has no code block example. This leaves it up to the user to guess whether calling the function sets yoyo to true or if a value needs to be passed in without referring to the code. Added lines to show that boolean values should be passed in to the function.

docs/user_guide.md

@@ -176,7 +176,12 @@ Check the [Repeat](../examples/08_repeat.html) example.
 
 ### `yoyo`
 
-This function only has effect if used along with `repeat`. When active, the behaviour of the tween will be _like a yoyo_, i.e. it will bounce to and from the start and end values, instead of just repeating the same sequence from the beginning.
+This function only has effect if used along with `repeat`. When active, the behaviour of the tween will be _like a yoyo_, i.e. it will bounce to and from the start and end values, instead of just repeating the same sequence from the beginning:
+
+```
+tween.yoyo(false) // default value, animation will only go from start to end value
+tween.yoyo(true) // tween will 'yoyo' between start and end values
+```
 
 ### `delay`