add case-insensitive note (dotnet#8064)

Author: marsen

xml/Microsoft.Extensions.Configuration/ConfigurationManager.xml

@@ -128,7 +128,7 @@
         <param name="key">To be added.</param>
         <summary>Gets a configuration subsection with the specified key.</summary>
         <returns>The configuration section for the specified key.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>The name of the configuration section is case-insensitive.</remarks>
       </Docs>
     </Member>
     <Member MemberName="Item">