ECL - How to set vision uncertainty (#587)

en/advanced_config/tuning_the_ecl_ekf.md

@@ -199,7 +199,7 @@ Position, velocity or orientation measurements from an external vision system, e
 * External vision system orientation data will be used for yaw estimation if bit position 4 in the [EKF2_AID_MASK](../advanced_config/parameter_reference.md#EKF2_AID_MASK) parameter is true.
 * External vision reference frame offset will be estimated and used to rotate the external vision system data if bit position 6 in the [EKF2_AID_MASK](../advanced_config/parameter_reference.md#EKF2_AID_MASK) parameter is true.
 
-Either bit 4 or bit 6 should be set to true.
+Either bit 4 (EV_YAW) or bit 6 (EV_ROTATE) should be set to true, but not both together.
 Following [EKF2_AID_MASK](../advanced_config/parameter_reference.md#EKF2_AID_MASK) values are supported when using with an external vision system.
 
 EKF_AID_MASK value | Set bits | Description
@@ -213,6 +213,8 @@ EKF_AID_MASK value | Set bits | Description
 280 | EV_POS + EV_VEL + EV_YAW | Heading w.r.t. external vision frame
 328 | EV_POS + EV_VEL + ROTATE_EV | Heading w.r.t. North
 
+The EKF considers uncertainty in the visual pose estimate. This uncertainty information can be sent via the covariance fields in the MAVLink [ODOMETRY](https://mavlink.io/en/messages/common.html#ODOMETRY) message or it can be set through the parameters [EKF2_EVP_NOISE](../advanced_config/parameter_reference.md#EKF2_EVP_NOISE), [EKF2_EVV_NOISE](../advanced_config/parameter_reference.md#EKF2_EVV_NOISE) and [EKF2_EVA_NOISE](../advanced_config/parameter_reference.md#EKF2_EVA_NOISE). You can choose the source of the uncertainty with [EKF2_EV_NOISE_MD](../advanced_config/parameter_reference.md#EKF2_EV_NOISE_MD).
+
 ## How do I use the 'ecl' library EKF?
 
 Set the [SYS_MC_EST_GROUP](../advanced_config/parameter_reference.md#SYS_MC_EST_GROUP) parameter to 2 to use the ecl EKF.