Add note on bug with `.index()` before 1.9

Added a little note on the bug with `.index()` in jquery < 1.9.

The examples were fixed with https://github.com/jquery/learn.jquery.com/commit/54946db1e37164f5f4154f6bded0236abbe6655c, where `.first()` was added without including any explanation about it. This results in even more confusion.

In general this needs to be known by people, so makes sense to include it.



Would fix https://github.com/jquery/learn.jquery.com/issues/276.