Document http "aborted" events

Checklist

• make -j4 test (UNIX) or vcbuild test nosign (Windows) passes (This PR just adds doc)
• a test and/or benchmark is included (This PR just adds doc)
• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

doc (for HTTP)

Description of change

This pull request follows on #6925 (closed) (in which @Fishrock123 and @dougwilson made key appearances) to document the "aborted" (as opposed to "abort") events emitted on HTTP requests and responses on socket closure by the remote system.

A few links from the issue that seem most relevant:

• HTTP client emitting 'aborted' from the corresponding response (req.res).
• HTTP server emitting 'aborted' from requests.
• The test for 'aborted'

The last tests the event on req from the server point of view. I actually don't see a test checking the event from the client API. This test sets a handler on a res from an http.get call, but as far as I can tell there's no test verifying its functionality. I can try and cook one up if needed.

_Be Warned! This is my first PR to Node.js. I reread CONTRIBUTING, but probably screwed something up anyway. Many thanks to team for patience and understanding!_

This PR coming to you live from doc sprint at NodeConf Adventure.