Improve description of deprecations

- Use a consistent format for all `v3` deprecations.
- Provide a recommendation for each `v3` deprecation.
- Move deprecation notice inline for `beta` /user/emails response format. This
  format will NOT be part of v3, so it doesn't belong in the v3 deprecations
  list.
Deprecate `forks`, `watchers`, & `open_issues` attributes

- Deprecate attributes and recommend preferred alternatives (which are already
  provided by API v3).
- Remove the deprecated attributes from the example responses. API v3 will
  continue to return `forks`, `watchers`, and `open_issues`, but we're no
  longer advertising them.
Merge pull request #350 from github/clarify-deprecations

Clarify list of expected API changes