Detail available space behavior on loop devices

Signed-off-by: Peter Volpe <petervo@redhat.com>

daemon/graphdriver/devmapper/README.md

@@ -62,10 +62,10 @@ status information about the driver.
  *  `Metadata file` blockdevice file used for the devicemapper metadata
  *  `Data Space Used` tells how much of `Data file` is currently used
  *  `Data Space Total` tells max size the `Data file`
- *  `Data Space Available` tells how much free space there is in the `Data file`
+ *  `Data Space Available` tells how much free space there is in the `Data file`. If you are using a loop device this will report the actual space available to the loop device on the underlying filesystem.
  *  `Metadata Space Used` tells how much of `Metadata file` is currently used
  *  `Metadata Space Total` tells max size the `Metadata file`
- *  `Metadata Space Available` tells how much free space there is in the `Metadata file`
+ *  `Metadata Space Available` tells how much free space there is in the `Metadata file`. If you are using a loop device this will report the actual space available to the loop device on the underlying filesystem.
  *  `Udev Sync Supported` tells whether devicemapper is able to sync with Udev. Should be `true`.
  *  `Data loop file` file attached to `Data file`, if loopback device is used
  *  `Metadata loop file` file attached to `Metadata file`, if loopback device is used