How to rename disk indentifier?
You've selected the Master Boot Record option. Only 4 primary partitions are allowed. disk2s5 and disk2s6 are actually extended partions. You cannot change the name conversion via DiskUtility.

If you select GUID Partiton Table option, you'll get this:

$ diskutil list disk2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Documents               200.0 GB   disk2s2
   3:                  Apple_HFS Downloads               200.0 GB   disk2s3
   4:                  Apple_HFS Movies                  200.0 GB   disk2s4
   5:                  Apple_HFS Music                   200.0 GB   disk2s5
   6:                  Apple_HFS Others                  199.2 GB   disk2s6