[Action-527] Propose spec change to address extensibility in general

Hi all,

I discussed with with David offline, and he agreed that we can close 
this finally by adding this sentence from
http://www.w3.org/International/multilingualweb/lt/drafts/its20/its20.html#its-schemas
"Foreign elements can be used only inside rules. Foreign attributes can 
be used on any element defined in ITS."
also to
http://www.w3.org/International/multilingualweb/lt/drafts/its20/its20.html#datacategories-defaults-etc

If nobody disagrees in the next day I will do the edit. We won't open a 
new issue here since it's just copying text.

Best,

Felix