Document how to set duration of playlist entries in API definition

Parent: [EPIC] Refactor playlist and metadata management (aura#155 - closed)

The Tank API for playlist entries behaves this way:

• When the playlist only holds a single item with duration, the duration is automatically expanded to the duration of the full timeslot
• When multiple items without duration information are present, an error is raised.

Proposal

Describe this behavior in the API docs.