Add example to System.Linq.AsyncEnumerable breaking change notice (#45861)

Author: stephentoub

docs/core/compatibility/core-libraries/10.0/asyncenumerable.md

@@ -42,7 +42,7 @@ If `System.Linq.Async` is consumed indirectly via another package, avoid ambigui
 </PackageReference>
 ```
 
-Most consuming code should be compatible without changes, but some call sites might need updates to refer to newer names and signatures.
+Most consuming code should be compatible without changes, but some call sites might need updates to refer to newer names and signatures. For example, a `Select` call like `e.Select(i => i * 2)` will work the same before and after. However, the call `e.SelectAwait(async (int i, CancellationToken ct) => i * 2)` will need to be changed to use `Select` instead of `SelectAwait`, as in `e.Select(async (int i, CancellationToken ct) => i * 2)`.
 
 Refer to the [System.Linq.AsyncEnumerable API documentation](xref:System.Linq.AsyncEnumerable) for the full set of LINQ extension methods available for <xref:System.Collections.Generic.IAsyncEnumerable`1>.