Clarify list of expected API changes

To get us one step closer to [ending beta mode on API v3][1]:

- Replace the "Expected Changes" section with a "Deprecations" section. Explain that the items in this section:
    - Are officially supported in API v3
    - Will be removed from the _next_ major version of the API
- Provide a recommendation for dealing with each deprecated feature.
- Remove deprecated attributes from response examples.
- Move the deprecation notice inline for the beta `/user/emails` response format. This format will _not_ be part of v3, so it doesn't belong in the v3 deprecations list.

/cc @pengwynn @gjtorikian @izuzak @jdennes 

[1]: https://github.com/github/developer.github.com/blame/5ecb6b72030cf4b3540d67958826488d57609026/content/index.md#L20-L22