Add section about `after` to special forms docs. (#4599)
Add section about `after` to special forms docs. (#4599)

Clarify that `after` in `try` is not guaranteed to run because an exit signal can abruptly stop the process. 

See https://groups.google.com/forum/#!topic/elixir-lang-core/NWvFwUWo2QQ for background conversation.
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