There is no way to add checkin comments. The standard WebDAV protocol simply doesn't support it. Fortress will add a "Checked in via WebDAV" comment with each checkin, but this is not the same as descriptive comments.

Checkins via WebDAV are non-atomic, and one-at-a-time. The base WebDAV protocol does not support changesets. You can still select a number of files or folders to check in at once, but within Fortress's history, they'll appear as a series of individual commits.