Doc: Do not call Greasemonkey attributes 'extensions'

The term stems from Chromium extensions, which are however overloaded.
Sadly there's no good page we can link to inside Chrome/Chromium for
the attributes we support ...

Change-Id: Ife7fcdba7bdfdbd4b7248b247c4baa188a44867a
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>