Adding a missing doc on the ObservationWriterExtensions (#4648)

com.unity.ml-agents/Runtime/Sensors/ObservationWriter.cs

         }
     }
 
+    /// <summary>
+    /// Provides extension methods for the ObservationWriter.
+    /// </summary>
-
         /// <summary>
         /// Writes a Texture2D into a ObservationWriter.
         /// </summary>