Improve docs on actions $unique and group parameters

[jorgeatorres]

Despite some improvements to the docblocks themselves, our current documentation doesn't clearly convey what unique actions mean or what the group parameter is for.

This stems from #1065:

as_schedule_single_action() & as_schedule_recurring_action() can accept a $group parameter to group different actions for the one plugin together.

This is a little unclear. What does it mean to "group actions together"? I can't seem to find anything explaining exactly what $group is for.

Edit: Same question with $unique. The documentation doesn't explain what this argument means. I'm guessing that you cannot enqueue a unique task if there is already one scheduled or in-progress?

It'd be nice if the API documentation includes a section explaining the semantics of the arguments.