The 3.2 docs claim that $comment is deprecated, I suspect because the find command has a "comment" option, rather than accepting $ operators:

https://docs.mongodb.com/manual/reference/operator/meta/comment/

But $comment is used more places than just find(). $comment can be used in any command that accepts query criteria (update, delete, findAndModify, aggregate, etc.). How to use $comment with these operations appears to be completely undocumented.