Improve runJavaScript documentation

Mention that the script will run in the same JavaScript world as scripts
that are part of the page. Also link to new scripts API.

Change-Id: I4dd80fdd84644b1fa4ce6c63ab07c1dbba855389
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>