[paymentrequest] Update API document with missing details (#38)

This pull request fills in some of the gaps created in the previous API refactoring change, fixes some bugs, and tidies up the document.
You can view, comment on, or merge this pull request online at:

  https://github.com/WICG/paymentrequest/pull/38

-- Commit Summary --

  * Tidying up SVG diagram
  * Added detail to algorithms and fixed shipping option bugs.
  * Updated document front matter.
  * Fixed typos and updated to US English spelling.

-- File Changes --

    M specs/paymentrequest.html (254)
    M specs/state-transitions.svg (80)

-- Patch Links --

https://github.com/WICG/paymentrequest/pull/38.patch
https://github.com/WICG/paymentrequest/pull/38.diff

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/paymentrequest/pull/38