Fix comments on CancellationTokenRegistration.Token (#4186)

Author: stephentoub

xml/System.Threading/CancellationTokenRegistration.xml

@@ -450,12 +450,17 @@ The returned <xref:System.Threading.Tasks.ValueTask> completes once the associat
         <ReturnType>System.Threading.CancellationToken</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets the <see cref="T:System.Threading.CancellationToken" /> with which this registration is associated.  If the
-            registration isn't associated with a token (such as after the registration has been disposed),
-            this will return a default token.</summary>
+        <summary>Gets the <see cref="T:System.Threading.CancellationToken" /> with which this registration is associated.</summary>
         <value>The cancellation token with which this registration is associated, or a default token if the
-            registration isn't associated with a token (such as after the registration has been disposed).</value>
-        <remarks>To be added.</remarks>
+            registration isn't associated with a token.</value>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+
+If the <xref:System.Threading.CancellationTokenRegistration> was created by registering with a <xref:System.Threading.CancellationToken> that already
+had cancellation requested, the registration may not be associated with the token, in which case this property may return a default token.
+
+         ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="Unregister">