[w3c/push-api] Editorial: cleanup + xrefs (#330)

Cleaned up a few little issues. 

Small but noticeable change: as there is no such thing anymore in WebIDL as "getting the underlying value" (and "getter steps" is not a concept exported from WebIDL [1]), I defined+linked-to the appropriate getter algorithms in the spec. 

[1] https://github.com/heycam/webidl/commit/ea3af2cfb2a8204ac17da27c347d760ae12b4d0a


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You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/push-api/pull/330

-- Commit Summary --

  * Editorial: cleanup + xrefs

-- File Changes --

    M index.html (47)

-- Patch Links --

https://github.com/w3c/push-api/pull/330.patch
https://github.com/w3c/push-api/pull/330.diff

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