[C#] Add ToString doc.

Author: mjpt777

csharp/sbe-dll/PrimitiveType.cs

@@ -208,6 +208,9 @@ public SbePrimitiveType Type
  get { return _sbePrimitiveType; }
  }
 
+ /// <summary>
+ /// The PrimitiveName as a string.
+ /// </summary>
  public override string ToString() => PrimitiveName;
  }
 }