fs: document mapping helpers

Document new helpers we introduced this cycle.

Link: https://lore.kernel.org/r/20210320122623.599086-2-christian.brauner@ubuntu.com
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>