document `license :trial`

This is styled as a PR, but intended for discussion.

Is `license :trial` useful?
* does it cover many Casks?
* can it be clearly and objectively defined?
* if clearly defined, is it also fully distinct from `:freemium`

If yes, we can document it, as it [already exists](https://github.com/caskroom/homebrew-cask/blob/d4f52efd11eee8e7be68220998ca56d29b5cc092/lib/cask/dsl/license.rb#L16).  If no, we should delete it from the code.

There is a third possible answer, that the name `:trial` was badly chosen, and that something else like `:limited` would be useful.  `:trial` could be considered part of DSL 1.0, as the forward-compatibility code is already present.  `:limited` or some other substitute would have to wait.

In any case, my priority is to maximize discussion here, and minimize decision-making per-Cask.

Refs #8017, #6426, #7917, #7923, #5586
cc @Amorymeltzer, @vitorgalvao, @ndr-qef