[media] cec: improve dqevent documentation

The documentation for the cec_event_state_change struct was incomplete.
This patch documents what happens in the corner cases.

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-dqevent.rst

@@ -64,7 +64,8 @@ it is guaranteed that the state did change in between the two events.
 
        -  ``phys_addr``
 
-       -  The current physical address.
+       -  The current physical address. This is ``CEC_PHYS_ADDR_INVALID`` if no
+          valid physical address is set.
 
     -  .. row 2
 
@@ -72,7 +73,10 @@ it is guaranteed that the state did change in between the two events.
 
        -  ``log_addr_mask``
 
-       -  The current set of claimed logical addresses.
+       -  The current set of claimed logical addresses. This is 0 if no logical
+          addresses are claimed or if ``phys_addr`` is ``CEC_PHYS_ADDR_INVALID``.
+	  If bit 15 is set (``1 << CEC_LOG_ADDR_UNREGISTERED``) then this device
+	  has the unregistered logical address. In that case all other bits are 0.