Document missing value in TimeSpan CompareTo EII (#4063)

Author: carlossanlop

xml/System/TimeSpan.xml

@@ -3717,7 +3717,7 @@ When a time interval component in the string to be parsed contains more than sev
         <Parameter Name="value" Type="System.Object" Index="0" FrameworkAlternate="dotnet-uwp-10.0;netcore-1.0;netcore-1.1;netstandard-1.0;netstandard-1.1;netstandard-1.2;netstandard-1.3;netstandard-1.4;netstandard-1.5;netstandard-1.6" />
       </Parameters>
       <Docs>
-        <param name="value">To be added.</param>
+        <param name="value">An object to compare, or <see langword="null" />.</param>
         <summary>Compares this instance to a specified object and returns an integer that indicates whether this instance is shorter than, equal to, or longer than the specified object.</summary>
         <returns>
           One of the following values: