media: cec-ioc-g-mode.rst: be more specific when EPERM is returned

Document which capability is required, rather than just saying
that "root permissions" are required.
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/cec/cec-ioc-g-mode.rst

@@ -294,7 +294,8 @@ EINVAL
     The requested mode is invalid.
 
 EPERM
-    Monitor mode is requested without having root permissions
+    Monitor mode is requested, but the process does have the ``CAP_NET_ADMIN``
+    capability.
 
 EBUSY
     Someone else is already an exclusive follower or initiator.