dm vdo: fix kerneldoc warnings

Fix kerneldoc warnings across the dm-vdo target. Also
remove some unhelpful or inaccurate doc comments, and fix
some format inconsistencies that did not produce warnings.

No functional changes.

Suggested-by: Sunday Adelodun <adelodunolaoluwa@yahoo.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>