Fix to JDK-8263155 left out the help text changes

The fix to JDK-8263155: Allow additional contents for DMG, left out the help text additions defined in it's CSR: JDK-8275148
The CSR says to add the new option:
  --mac-dmg-content <path to additional content>[,<path to additional content>...]
    Include all the referenced content in the dmg.
    This option can be used multiple times.