Add explanatory comment

src/EditorFeatures/Core/Implementation/IntelliSense/SignatureHelp/Presentation/Signature.cs

@@ -68,6 +68,9 @@ public IParameter CurrentParameter
             }
         }
 
+        /// <remarks>
+        /// The documentation is included in <see cref="Content"/> so that it will be classified.
+        /// </remarks>
         public string Documentation => null;
 
         private ReadOnlyCollection<IParameter> _parameters;