fs/namespace: correct/improve kernel-doc notation

fs/namespace: correct/improve kernel-doc notation

Fix kernel-doc warnings in fs/namespace.c:

./fs/namespace.c:1379: warning: Function parameter or member 'm' not described in 'may_umount_tree'
./fs/namespace.c:1379: warning: Excess function parameter 'mnt' description in 'may_umount_tree'
./fs/namespace.c:1950: warning: Function parameter or member 'path' not described in 'clone_private_mount'

Also convert path_is_mountpoint() comments to kernel-doc.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Allegedly-acked-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20210318025227.4162-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>