Explain how HTTP header fields integrate with URL patterns

[jeremyroman]

Per discussion on #182 some text explaining this would be useful.

This mostly consists of advice since the useful algorithms are already exposed.

• [x] At least two implementers are interested (and none opposed):
  • Chromium
  • WebKit
• [x] Tests are written and can be reviewed and commented upon at:
  • n/a (tests would apply to any such header, not to this text itself)
• [x] Implementation bugs are filed: n/a
• [x] MDN issue is filed: n/a
• [x] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

Preview | Diff

[jeremyroman]

@domenic @annevk does this address the concern in https://github.com/whatwg/urlpattern/issues/182#issuecomment-2313747316?