doc: include V8 commit URL in V8 backport guide

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

Bikeshedding over Commit: welcome.

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

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, v8

cc/ @nodejs/v8