Add note on bug with .index() before 1.9

Author: arthurvr

page/using-jquery-core/understanding-index.md

@@ -40,6 +40,8 @@ In the first example, `.index()` gives the zero-based index of `#foo1` within it
 
 When `.index()` is called on a jQuery object that contains more than one element, it calculates the index of the first element.
 
+Before jQuery 1.9 you should ensure `.index()` is only passed one element. Due to a bug that existed then, when you pass more its result may be incorrect. That's also the reason we're calling `.first()` every time we use `.index()` in our example.
+
 ## `.index()` with a String Argument
 
 ```