Document the meaning of bare suicide triggers.

Closes #835.

@arjclark - please review.

[EDIT: this pull request was rebased, it's title changed, and some comments removed, so the post-rebase version makes sense if consulted in the future.   This reflects a change of tack from disallowing bare suicide triggers to documenting their meaning.  (I should perhaps have just cancelled the original pull request and issued a new one...)]