Improve and standardise event docs

Having the general events description in events.condition.jsdoc isn't very logical, but this would resolve itself if #2334 goes ahead.
I still do not understand this sentence. I hope it's not just me :-)

I'd write something like this: "In principle the application code need not itself create event instances. The application code registers event listeners (which are functions, really), which, when called by the library, are passed event instances as their first arguments.". I find this more explicit and concrete.