Mark internal properties in FastBlur documentation.

Change-Id: Icebfba44bcf22e851b925f05546d46b26d4d5cae
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: If1557686dd6626724246f700e78f77f4ed7d6d0c