Document the "Start thread" alternative to "Comment"

This is part of the "update the contributors doc" task (#30 (closed)).

Long discussions on issue trackers are typically hard to track, in particular when multiple topics and subtopics are discussed: it's hard to tell on which ones we reached a conclusion yet, and which ones are still pending. https://redmine.tails.boum.org/code/issues/15186 and my experience in GitLab on other projects show that people who discover GitLab tend to click "Comment" (quite understandably as it is the default), in many cases when they should really click "Start thread", because such a thread can later be marked as resolved.

I trust this can be solved by a combination of documentation, learning by example, and gentle mentoring as folks get started with GitLab. This issue is about the documentation aspect.

Note that if that does not get solved, the end result is just as bad, and no worse, than what we have on Redmine.