Update Assembly.xml (#4979)

Author: gewarren

xml/System.Reflection/Assembly.xml

@@ -3315,10 +3315,20 @@ In .NET 5.0 and later versions, for bundled assemblies, this method throws an ex
         <ReturnType>System.Boolean</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets a value indicating whether the assembly was loaded from the global assembly cache.</summary>
+        <summary>Gets a value indicating whether the assembly was loaded from the global assembly cache (.NET Framework only).</summary>
         <value>
-          <see langword="true" /> if the assembly was loaded from the global assembly cache; otherwise, <see langword="false" />.</value>
-        <remarks>To be added.</remarks>
+          .NET Framework only: <see langword="true" /> if the assembly was loaded from the global assembly cache; otherwise, <see langword="false" />.
+        
+          .NET Core and .NET 5.0 and later: <see langword="false" /> in all cases.</value>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+This property is marked obsolete starting in .NET 5.0, and generates a compile-time warning.
+
+ ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="HostContext">