ext4: Orphan file documentation

Add documentation about the orphan file feature.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210816095713.16537-4-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>