doc: include V8 commit URL in V8 backport guide

Use `Commit:` for the V8 commit, and `PR-URL:` for the Node PR URL.

Refs: https://github.com/nodejs/node/pull/16053#issuecomment-334868621
PR-URL: https://github.com/nodejs/node/pull/16054


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>