Clarified documentation on null case.

Source/SvgDocument.cs

@@ -422,7 +422,7 @@ namespace Svg
         /// Renders the <see cref="SvgDocument"/> to the specified <see cref="Graphics"/>.
         /// </summary>
         /// <param name="graphics">The <see cref="Graphics"/> to be rendered to.</param>
-        /// <param name="size">The <see cref="SizeF"/> to render the document.</param>
+        /// <param name="size">The <see cref="SizeF"/> to render the document. If <c>null</c> document is rendered at the default document size.</param>
         /// <exception cref="ArgumentNullException">The <paramref name="graphics"/> parameter cannot be <c>null</c>.</exception>
         public void Draw(Graphics graphics, SizeF? size = null)
         {