TIF: update documentation of TvInputInfo.getParentId()

    /**

     * Returns the parent input ID.

     * <p>

     * When a part of the functionalities of a TV input is actually provided by another TV input,

     * we can describe this relationship as the depending input having a "parent". It is primarily

     * used for controlling underlying hardware when the current input itself does not have direct

     * access to it. Examples include a TV input for a specific HDMI CEC logical device having a

     * generic HDMI input as its parent and a HDMI-paired virtual input whose video stream comes

     * from an external settop box. Applications may group inputs by parent ID to provide an easier

     * access to similar inputs.

     * A TV input may have a parent input if the TV input is actually a logical representation of

     * a device behind the hardware port represented by the parent input.

     * For example, a HDMI CEC logical device, connected to a HDMI port, appears as another TV

     * input. In this case, the parent input of this logical device is the HDMI port.

     * </p><p>

     * Applications may group inputs by parent input ID to provide an easier access to inputs

     * sharing the same physical port. In the example of HDMI CEC, logical HDMI CEC devices behind

     * the same HDMI port have the same parent ID, which is the ID representing the port. Thus

     * applications can group the hardware HDMI port and the logical HDMI CEC devices behind it

     * together using this method.

     * </p>

     *

     * @return the ID of the parent input, if exists. Returns {@code null} if the parent input is

     *         not specified.