fs: add kernel doc for mnt_{hold,unhold}_writers()

When I introduced mnt_{hold,unhold}_writers() in commit fbdc2f6c
("fs: split out functions to hold writers") I did not add kernel doc for
them. Fix this and introduce proper documentation.

Link: https://lore.kernel.org/r/20220203131411.3093040-4-brauner@kernel.org
Fixes: fbdc2f6c

 ("fs: split out functions to hold writers")
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>