docs: Clarify Trial Status and One-Time Purchase Trials

[rishi-raj-jain]

From the ticket

The current API documentation is outdated regarding trial status and related fields. For example, documentation references a "trialing" status that is not present in the status field values. It is unclear how trial status is represented for existing contracts and how potential new fields in the license key validation response work. This lack of clarity makes it difficult to understand how trial affects license key validation and how to implement or support these workflows.