[w3c/push-api] Mention how applicationServerKey is used (#250)

Builds on #249, which you can merge to see the changes in this PR more clearly.

Closes #209.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/push-api/pull/250

-- Commit Summary --

  * Run tidy
  * Improve definition for applicationServerKey

-- File Changes --

    M index.html (441)

-- Patch Links --

https://github.com/w3c/push-api/pull/250.patch
https://github.com/w3c/push-api/pull/250.diff

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https://github.com/w3c/push-api/pull/250