clarify the definition of "navigation scope", "applied", and off-scope theming

[marcoscaceres]

Closes #880

This is @mgiuca original commit.

Closes #755.

This change (choose one):

• Breaks existing normative behavior (please add label "breaking")
• Adds new normative requirements
• Adds new normative recommendations or optional items
• Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
• Is a "chore" (metadata, formatting, fixing warnings, etc).

Commit message:

This further clarifies that a manifest is allowed to apply to URLs that
are not within its scope (that this was not clear was a left-over
mistake from #701), and that a document within scope is allowed to
override the theme color. Adds a new recommendation that an out of scope
document not be allowed to override the theme color.

This introduces new normative recommendations, only insofar as they were
already commonly understood, to my knowledge.

Navigation scope: No longer defined in terms of the URLs that a manifest
is allowed to apply to (since a manifest may apply to any URL). Rather,
it's just a set of URLs; the other normative requirements around
navigation scope define what it means.

Applied: Clarified that it is the user agent's discretion whether or not
to apply a manifest when a top-level browsing context is created, with
examples of when you would and would not apply it (previously, there was
simply no text around when a manifest should be applied).

Theme color: Fixes a "MAY" requirement imposed on the document
(requirements can only be imposed on user agents). Clarifies this
recommendation as only applying to in-scope documents, and adds a
recommendation against applying document theme for out-of-scope
documents.

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[dmurph]

LGTM

[mgiuca]

Thanks for cleaning up and landing one of my old PRs!