Update help manual for support of devices without partition tables (#743181)

GParted now recognizes file system formats on disk devices without
partition tables.  Update the manual with:

  - how to format a disk device without a partition table
  - manage flags not available for devices without partition tables
  - use format to cleared to delete a file system from an unpartitioned
    disk

Bug 743181 - Add unpartitioned drive read-write support