Update to Key Discovery draft

All,

I have pushed an update to the Key Discovery draft at:
https://dvcs.w3.org/hg/webcrypto-keydiscovery/raw-file/tip/Overview.html

The changes are small, to align with TAG recommended terminology for
promises, to remove the reference to the now-closed Issue 25, to provide
links for submitting bugs (presently pointing at the API component, since a
Key Discovery component was never created - I raised this with Harry and
will update when it is done) to add an example and a few minor fixes for
links etc.

This will need a further update to align with whatever name we give the Key
object in the main API document, but I think it is ready for Last Call.

...Mark