[docs] Add missing package docs (#5108)

* Add missing package docs

* Fix grammar

* Update comment on Dispose

* Fix typo in doc

com.unity.ml-agents/Runtime/IMultiAgentGroup.cs

         /// <summary>
         /// Register agent to the MultiAgentGroup.
         /// </summary>
+        /// <param name="agent">The Agent to register.</param>
+        /// <param name="agent">The Agent to unregister.</param>
         void UnregisterAgent(Agent agent);
     }
 }

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

                 DimensionProperty.VariableSize,
                 DimensionProperty.None
             };
+
+        /// <summary>
+        /// Creates the BufferSensor.
+        /// </summary>
+        /// <param name="maxNumberObs">The maximum number of observations to be appended to this BufferSensor.</param>
+        /// <param name="obsSize">The size of each observation appended to the BufferSensor.</param>
+        /// <param name="name">The name of the sensor.</param>
         public BufferSensor(int maxNumberObs, int obsSize, string name)
         {
             m_Name = name;

com.unity.ml-agents/Runtime/SimpleMultiAgentGroup.cs

         readonly int m_Id = MultiAgentGroupIdCounter.GetGroupId();
         HashSet<Agent> m_Agents = new HashSet<Agent>();
 
-
+        /// <summary>
+        /// Disposes of the SimpleMultiAgentGroup.
+        /// </summary>
         public virtual void Dispose()
         {
             while (m_Agents.Count > 0)