Update CancellationTokenSource.xml (dotnet#3982)

Add note to Dispose documentation to highlight that the `CancellationTokenSource` does not cancel its underlying `CancellationToken` when it is disposed, and that cancellation must be triggered separately.

Author: wazzamatazz

xml/System.Threading/CancellationTokenSource.xml

@@ -710,7 +710,9 @@
   
 ## Remarks  
  Call `Dispose` when you are finished using the <xref:System.Threading.CancellationTokenSource>. The `Dispose` method leaves the <xref:System.Threading.CancellationTokenSource> in an unusable state. After calling `Dispose`, you must release all references to the <xref:System.Threading.CancellationTokenSource> so the garbage collector can reclaim the memory that the <xref:System.Threading.CancellationTokenSource> was occupying.  
-  
+
+ Note that calling `Dispose` does not communicate a request for cancellation to consumers of the associated <xref:System.Threading.CancellationTokenSource.Token%2A>. You can communicate a request for cancellation by calling methods such as <xref:System.Threading.CancellationTokenSource.Cancel%2A> or <xref:System.Threading.CancellationTokenSource.CancelAfter%2A>. 
+
  For more information, see [Cleaning Up Unmanaged Resources](~/docs/standard/garbage-collection/unmanaged.md) and [Implementing a Dispose Method](~/docs/standard/garbage-collection/implementing-dispose.md).  
   
 > [!NOTE]