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

This removes a mention to OnceCallback support of PostTask, since
PostTask does support OnceCallback now.

BUG=554299

Review-Url: https://codereview.chromium.org/2830223002
Cr-Commit-Position: refs/heads/master@{#466591}

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));
 }
 ```