[media] cec-ioc-adap-g-caps.rst: document CEC_CAP_NEEDS_HPD

Document the new CEC_CAP_NEEDS_HPD capability.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/cec/cec-ioc-adap-g-caps.rst

@@ -113,6 +113,14 @@ returns the information to the application. The ioctl never fails.
       - 0x00000020
       - The CEC hardware can monitor all messages, not just directed and
 	broadcast messages.
+    * .. _`CEC-CAP-NEEDS-HPD`:
+
+      - ``CEC_CAP_NEEDS_HPD``
+      - 0x00000040
+      - The CEC hardware is only active if the HDMI Hotplug Detect pin is
+        high. This makes it impossible to use CEC to wake up displays that
+	set the HPD pin low when in standby mode, but keep the CEC bus
+	alive.