Clarify that the CompleteWithin works on Func<Task> and not on Task directly.

IMPORTANT

• The code complies with the Coding Guidelines for C#.
• The changes are covered by a new or existing set of unit tests which follow the Arrange-Act-Assert syntax such as is used in this example.
• If the contribution affects the documentation, please include your changes to documentation.md in this pull request so the documentation will appear on the website.