AP_NavEKF3: add enumeration to document EKF SolutionStatus

this isn't used for anything but documenting the solution status field, which can be used in the Wiki and in various log analysis tools

libraries/AP_NavEKF3/LogStructure.h

@@ -188,6 +188,7 @@ struct PACKED log_XKF3 {
 // @Field: FS: Filter fault status
 // @Field: TS: Filter timeout status bitmask (0:position measurement, 1:velocity measurement, 2:height measurement, 3:magnetometer measurement, 4:airspeed measurement, 5:drag measurement)
 // @Field: SS: Filter solution status
+// @FieldBitmaskEnum: SS: NavFilterStatusBit
 // @Field: GPS: Filter GPS status
 // @Field: PI: Primary core index
 struct PACKED log_XKF4 {