Add library doc comment to dart:cli

Needed to fix broken build:
https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8832225116390642513/+/u/build_api_docs/stdout

Cq-Include-Trybots: luci.dart.try:dart-sdk-linux-try
Change-Id: I9d1b5a94781de4a4380aebf5ccbabbb0d31a75d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/218420
Reviewed-by: Alexander Thomas <athom@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Michael Thomsen <mit@google.com>
diff --git a/sdk/lib/cli/cli.dart b/sdk/lib/cli/cli.dart
index cb260d9..f43efeb 100644
--- a/sdk/lib/cli/cli.dart
+++ b/sdk/lib/cli/cli.dart
@@ -2,6 +2,13 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+/// Utilities for building CLI apps.
+///
+/// ## Deprecation notice
+///
+/// The functionality of this library is incomplete and may be removed in a
+/// later version. See [waitFor] for details.
+///
 /// {@category VM}
 @Deprecated(
     "The functionality of this library is incomplete and may be removed in a later version")