Update plugin development documentation for v8

[jodeleeuw]

Things to change:

• Mention that trial can be async and return the data rather than calling jsPsych.finishTrial()

[jodeleeuw]

@bjoluc are there any other major changes here to update in the plugin documentation that I'm missing?

[bjoluc]

@jodeleeuw If we decide to drop pretty_name, that would be one thing. If we decide to accept undefined parameters without a default value and introduce required: true instead, that would be the other thing. And #3241 of course, if we go that route.