ACTION-1489 updated

I took a second pass at implementing ACTION-1489:

https://rawgit.com/w3c/aria/ACTION-1489/aria/aria.html#host_general_conflict

The last two paragraphs of the section represent the action to say 
"don't go to town with stopping overriding native semantics". I was 
asked to include wording to specifically say aria-label may not be 
overridden; I added aria-labelledby and aria-describedby but don't know 
for sure if they need to be listed, or if anything else does.

Michael