btrfs-progs: mkfs.btrfs documentation: clarify current restrictions of sectorsize...

btrfs-progs: mkfs.btrfs documentation: clarify current restrictions of sectorsize, nodesize and leafsize

Commit 8d082fb727ac11930ea20bf1612e334ea7c2b697 (Btrfs: do not mount when
we have a sectorsize unequal to PAGE_SIZE) requires the sectorsize to be
equal to the pagesize for the filesystem to be mountable.

The nodesize and leafsize should be equal, and not larger than 65536.

Adding this information to the manpage and usage instructions of mkfs.btrfs.

Signed-off-by: Koen De Wit <koen.de.wit@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>