UPGRADE: Clarify cross-origin subresource upgrades.

[mikewest]

@diracdeltas notes in w3ctag/design-reviews#54 that:

ISSUE: Same-Origin vs Cross-Origin Behavior Unclear in Examples

Talking to other TAG members about the spec, it became apparent that some of us
thought the spec only applied upgrades to same-origin requests. I attribute
most of my confusion to the examples in Section 1.2. Example #1 uses the
example of <img src="http://example.com/image.png"> being upgraded on
https://example.com and Example #2 explicitly says that <a href="http://not-example.com/">Home</a> will not be upgraded on
https://example.com. It would be better if Example #1 explicitly said that
a third-party origin like not-example.com is upgradeable in that context, so
that readers don't generalize Example #2 to all requests.