doc: document clientRequest.aborted

Add documentation for http clientRequest.aborted.

PR-URL: https://github.com/nodejs/node/pull/11544
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/http.md

@@ -450,6 +450,14 @@ added: v0.3.8
 Marks the request as aborting. Calling this will cause remaining data
 in the response to be dropped and the socket to be destroyed.
 
+### request.aborted
+<!-- YAML
+added: v0.11.14
+-->
+
+If a request has been aborted, this value is the time when the request was
+aborted, in milliseconds since 1 January 1970 00:00:00 UTC.
+
 ### request.end([data][, encoding][, callback])
 <!-- YAML
 added: v0.1.90