Draft text for authenticated origins

At TPAC we agreed I would come up with some text to encourage people to
look at authenticated origins for gUM without mandating it. Domenic
contributed some
text in Bugzilla (thanks) and I've modified it some to something I'm happy
with, as
below. PTAL:

When on an insecure origin, user agents are encouraged wish to warn
about usage of MediaDevices.getUserMedia, navigator.getUserMedia, and any
prefixed variants in their developer tools, error logs, etc. It is
explicitly
permitted for user agents to remove these APIs entirely when on an
insecure origin, as long as they remove all of them at once (e.g.,
they should not leave just the prefixed version available on insecure
origins).