Update SortDescription.xml

I added that if someone uses empty string as the value for parameter propertyName.

Author: Omcsesz

xml/System.ComponentModel/SortDescription.xml

@@ -39,7 +39,7 @@
         <Parameter Name="direction" Type="System.ComponentModel.ListSortDirection" />
       </Parameters>
       <Docs>
-        <param name="propertyName">The name of the property to sort the list by.</param>
+        <param name="propertyName">The name of the property to sort the list by. Should be set to "" if the collection to be sorted contains strings, integers, etc..</param>
         <param name="direction">The sort order.</param>
         <summary>Initializes a new instance of the <see cref="T:System.ComponentModel.SortDescription" /> structure.</summary>
         <remarks>To be added.</remarks>