bigalloc cluster size

Hello,

I've noted that valid cluster size is not limited by 256M according to "mke2fs" manual:

"-C cluster-size
Specify the size of cluster in bytes for filesystems using the bigalloc feature. Valid cluster-size values are from 2048 to 256M bytes per cluster."

but actually it's limited by 512M:

"[root@fedora ~]# mkfs.ext4 -O bigalloc -C 512m /dev/sdb mke2fs 1.43-WIP (29-Mar-2015)
/dev/sdb contains a ext4 file system

    created on Tue Apr 21 15:30:27 2015

Proceed anyway? (y,n) yes

Warning: the bigalloc feature is still under development
See https://ext4.wiki.kernel.org/index.php/Bigalloc for more information

Creating filesystem with 2097152 4k blocks and 16 inodes

Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

[root@fedora ~]# mkfs.ext4 -O bigalloc -C 1024m /dev/sdb mkfs.ext4: invalid cluster size - 1024m"

(running on vbox Fedora x86_64, latest e2fsprogs, kernel 4.0.0-rc5)

Is it documentation issue?

Thanks,
Alexander Tsvetkov