Update docs for IResourceRequestHandlerFactory (#3998)

I have just discovered that when HasHandlers returns false, the GetResourceRequestHandler method is never executed.  Presumably this is an optimisation which happens inside of Chromium, as I can see that the HasHandlers property value goes all way back to the C++ native project.

I was about to suggest removing this optimisation because it'd be trivial to optimise and return early inside the default factory impl in C# world.  If it goes all the way down to Chromium then that's less easy and so the next best thing would be to document it.

CefSharp/IResourceRequestHandlerFactory.cs

@@ -16,6 +16,13 @@ public interface IResourceRequestHandlerFactory
         /// <summary>
         /// Are there any <see cref="ResourceHandler"/>'s registered?
         /// </summary>
+        /// <remarks>
+        /// <para>
+        /// Implementors must return <see langword="true"/> from this method if the factory is to be used.
+        /// The <see cref="GetResourceRequestHandler"/> method will not be executed if this property returns
+        /// <see langword="false"/>, presumably as an optimisation.
+        /// </para>
+        /// </remarks>
         bool HasHandlers { get; }
 
         /// <summary>