doc: document response.finished in http.markdown

Adds mention of response.finished to http.markdown
Originally submitted by @hackerjs. The original
commit needed a bit of cleanup on grammar.

Reviewed By: James M Snell <jasnell@gmail.com>
Reviewed By: Michaël Zasso <mic.besace@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2414