[css-values] `url()`, cacheing, and incognito mode

[svgeesus]

Originally raised over here during security and privacy review for downloadable ICC color profiles. But it applies to all resources (images, etc) in CSS so is best documented on url() (and on any successor to url() be that called fetch() or src() or whatever)

The definition of the url() function should clarify that

the referenced file is persisted in your browser cache per standard resource caching rules, and is cleared in the same way.

[fantasai]

@svgeesus We added a note about caching, let us know if this is satisfactory.

[svgeesus]

LGTM