Add small in-code documentation

com.unity.render-pipelines.high-definition/HDRP/Lighting/Reflection/Volumes/ProxyVolume.cs

         [SerializeField, Obsolete("Kept only for compatibility. Use m_Shape instead")]
         bool m_SphereInfiniteProjection = false;
 
+        /// <summary>The shape of the proxy</summary>
+        /// <summary>The size of the proxy if it as a shape Box</summary>
+        /// <summary>The radius of the proxy if it as a shape Sphere</summary>
         public float sphereRadius { get { return m_SphereRadius; } set { m_SphereRadius = value; } }
 
 

com.unity.render-pipelines.high-definition/HDRP/Lighting/Reflection/Volumes/ReflectionProxyVolumeComponent.cs

         [SerializeField]
         ProxyVolume m_ProxyVolume = new ProxyVolume();
 
+        /// <summary>Access to proxy volume parameters</summary>
         public ProxyVolume proxyVolume { get { return m_ProxyVolume; } }
 
         void ISerializationCallbackReceiver.OnBeforeSerialize()