Update //docs/callback.md for OnceCallback support of PostTask impls

docs/callback.md

Index: docs/callback.md
diff --git a/docs/callback.md b/docs/callback.md
index 4194d5107995219c4dc65919040635eddd42cc0c..5f93ad6afa742fc405b5dc5cc2010fca333a2318 100644
--- a/docs/callback.md
+++ b/docs/callback.md
@@ -67,11 +67,9 @@ void Baz(OnceCallback<void(int)> cb) {
 
 // |Qux| takes the ownership of |cb| and transfers ownership to PostTask(),
 // which also takes the ownership of |cb|.
-// NOTE: TaskRunner is not actually migrated to OnceClosure yet. Once TaskRunner
-// supports OnceClosure, a OnceCallback can be posted as follows:
 void Qux(OnceCallback<void(int)> cb) {
   PostTask(FROM_HERE,
-           base::BindOnce(std::move(cb), 42));  // not yet implemented!
+           base::BindOnce(std::move(cb), 42));
 }
 ```