[CSS] cssref contains incorrect and insufficient information about url

Instead of partially fixed RT-21511:
"<URI> section of cssref.html has the following problems:
1. It doesn't specify what is the relative location in case of setStyle() usage. My experiments show that it is relative to the classes root (Not the particular class package which one may expect).
2. It doesn't specify what is the root (if you start your URI with "/" symbol). For example, when url('img/JavaFX.png') works url('/img/JavaFX.png') doesn't work. "