RuntimeInformation.OSDescription: add intended use-case. (#8968)

Author: tmds

xml/System.Runtime.InteropServices/RuntimeInformation.xml

@@ -244,8 +244,18 @@ The returned value is intended to represent the actual architecture of the under
       </ReturnValue>
       <Docs>
         <summary>Gets a string that describes the operating system on which the app is running.</summary>
-        <value>The description of the operating system on which the app is running.</value>
-        <remarks>To be added.</remarks>
+        <value>An opaque string that describes the operating system on which the app is running.</value>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+This property returns a string that describes the operating system for presentation to the user.
+
+It's not recommended to parse the string for information.
+
+          ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="ProcessArchitecture">