Add DocCommentingGuide

Tips for applying comments and track-changes during collaborative document reviews.

• Addresses: templates#378 (closed)
• Source Google Doc: https://docs.google.com/document/d/1x4-UUjP-ZSs8XqeK2puO_lLALHcYKMiLbRpz3cLrOSM
• This document has gone through Community and Editorial review.