getBudget should return a lower bound

[johnmellor]

getBudget should be documented as returning the amount of budget that there is expected to be assuming the user doesn't interact with the site at all in the meantime (i.e. a weak lower bound, though it's possible the budget will go below the prediction if e.g. the user revokes a permission for the website).

Otherwise UAs can't make reasonable predictions, and getBudget would be less actionable for developers.