That is what the "Description" tab is for. However, we recognize that not everyone will want to use the text area for this and may prefer a more formal approach. In which case, we recommend the use of MS Word and attaching the file as an attachment to the work item.
I'm sure we do things backwards here, but let me give you a peek at what we do. Our data analysts create the functional requirements. The developers then take those functional requirements and fill out how they will meet the requirement in a "technical requirement" tab that's linked to the functional requirement. Do these technical requirements lend themselves as tasks in VSTS?
Functional Requirements - What to do.
Technical Requirements - How to do it. (Task goes here?)