improved parameter doc

com.unity.perception/Documentation~/Randomization/Parameters.md

-# Custom Parameters
+# Parameters
+
+## Lookup Parameters
+
+To obtain a parameter from a paramter configuration, use the GetParameter() method:
+```
+// Get a reference to the parameter configuration attached to this GameObject
+var parameterConfiguration = GetComponent<ParameterConfiguration>();
+
+// Lookup the parameter "ObjectWidth" by name
+var parameter = GetComponent<FloatParameter>("ObjectWidth");
+``` 
+
+## Creating and Sampling Parameters
+
+Parameters are typically managed by `ParameterConfigurations` in the Unity Editor. However, parameters can be instanced independently like a regular class too:
+```
+// Create a color parameter
+var colorParameter = new HsvaColorParameter();
+
+// Generate one color sample
+var color = colorParameter.Sample();
+```
+
+Note that parameters, like samplers, generate new random values for each call to the Sample() method:
+```
+var color1 = colorParameter.Sample();
+var color2 = colorParameter.Sample();
+Assert.AreNotEqual(color1, color2);
+```
+
+## Defining Custom Parameters
-All parameters derive from the Parameter abstract class, but all included perception package parameter types derive from two specialized Parameter base classes:
+All parameters derive from the `Parameter` abstract class, but all included perception package parameter types derive from two specialized Parameter base classes:
 1. `CategoricalParameter`
 2. `NumericParameter`
 
 
 Numeric parameters use samplers to generate randomized structs. Take a look at the [ColorHsvaParameter]() class included in the perception package for an example on how to implement a numeric parameter.
 
-## Performance
+## Improving Sampling Performance
+// Get a reference to the parameter configuration attached to this GameObject
+var parameterConfiguration = GetComponent<ParameterConfiguration>();
+
+// Lookup parameters
+var cubeColorParameter = parameterConfiguration.GetParameter<HsvaColorParameter>("CubeColor");
+var cubePositionParameter = parameterConfiguration.GetParameter<Vector3Parameter>("CubePosition");
+
-var currentIteration = ScenarioBase.ActiveScenario.currentIteration
-var cubeColors = ObjectColor.Samples(constants.objectCount, out var colorHandle);
-var cubePositions = ObjectPosition.Samples(constants.objectCount, out var positionHandle);
+var cubeColors = cubeColorParameter.Samples(constants.cubeCount, out var colorHandle);
+var cubePositions = cubePositionParameter.Samples(constants.cubeCount, out var positionHandle);
 
 // Combine job handles
 var handles = JobHandle.CombineDependencies(colorHandle, positionHandle);
 
 // Use the created samples
-for (var i = 0; i < constants.objectCount; i++)
+for (var i = 0; i < constants.cubeCount; i++)
 {
     m_ObjectMaterials[i].SetColor(k_BaseColorProperty, cubeColors[i]);
     m_Objects[i].transform.position = cubePositions[i];

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

 using System;
 using System.Collections.Generic;
+using Unity.Assertions;
 using UnityEngine.Perception.Randomization.Samplers;
 
 namespace UnityEngine.Perception.Randomization.Parameters