Consistently use URI (in place of URL)

[Kyle Larose]

Multiple reviewers have suggested that we use URI instead of URL.

But that's backward: the IETF formally defines "URI" [RFC3986], not
"URL", and that document says [Section 1.1.3]:

Future specifications and related documentation should
use the general term "URI" rather than the more restrictive terms
"URL" and "URN"

(Additionally, we should probably include an informative reference to
3986 on first use of the term.)

I think we should make this change, and add the informative reference.