added ParameterApplicationFrequency xmldoc

com.unity.perception/Runtime/Randomization/Parameters/ParameterTarget.cs

     /// </summary>
     public enum ParameterApplicationFrequency
     {
+        /// <summary>
+        /// Applies a parameter once every iteration
+        /// </summary>
+
+        /// <summary>
+        /// Applies a parameter once every frame
+        /// </summary>
         EveryFrame
     }
 

com.unity.perception/Runtime/Randomization/Parameters/Attributes.meta

-fileFormatVersion: 2
-guid: 61fd0d1421574ffd9f04d82f82744743
-timeCreated: 1594662190