propagate_mnt(): fix comment and convert to kernel-doc, while we are at it

Mountpoint is passed as struct mountpoint *, not struct dentry *
(and called dest_mp, not dest_dentry) since 2013.

Roots of created copies are linked via mnt_hash, not mnt_list since
a bit before the merge into mainline back in 2005.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>