Document mode is both a style of topic structure, and a technique of writing those topics.
Problem: You need to record/communicate factual information or reference material.
Solution: Document mode is essentially the standard presentation of material one finds in books and articles, tweaked slightly for wikis.
Document mode topics are generally more easily understood by novice wiki users (they're not used for online discussions).
So, use Document mode.
If you're converting a discussion mode topic to a document mode topic: