Add heading for response formats for pull requests
Explain and deprecate the merge_commit_sha

The merge_commit_sha is often misunderstood. It is internal GitHub
plumbing that just happens to be part of the public API. Explain it
(to reduce the confusion for the current version of the API), and
announce plans to remove it from the next version of the API.
Link merge_commit_sha explanation to deprecation
Improve "mergability" docs

Thanks to @gjtorikian for the suggestion.
Merge pull request #236 from github/deprecate-merge-commit-sha

Explain & deprecate merge_commit_sha