[media] cec-core.rst: document the new cec_get_drvdata() helper

Document the new cec_get_drvdata() helper function.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/cec-core.rst

@@ -48,6 +48,7 @@ ops:
 
 priv:
 	will be stored in adap->priv and can be used by the adapter ops.
+	Use cec_get_drvdata(adap) to get the priv pointer.
 
 name:
 	the name of the CEC adapter. Note: this name will be copied.
@@ -62,6 +63,10 @@ available_las:
 	the number of simultaneous logical addresses that this
 	adapter can handle. Must be 1 <= available_las <= CEC_MAX_LOG_ADDRS.
 
+To obtain the priv pointer use this helper function:
+
+.. c:function::
+	void *cec_get_drvdata(const struct cec_adapter *adap);
 
 To register the /dev/cecX device node and the remote control device (if
 CEC_CAP_RC is set) you call: