Fix #1802; Update docs to avoid misconceptions around emitting events

[shentao]

No description provided.

[sdras]

I think this is much better. But I wonder if we should take it a step further (not within this PR). What if I made a little animation for this? Kind of like the two videos here: https://css-tricks.com/understanding-the-almighty-reducer/

There's something about not being able to see the replacement happening and instead reading sequentially that I think is causing a cognitive overload. What do you both think?

I do approve this PR though, and think it's a great step. Curious what Chris thinks here too.

[shentao]

I think that having a diagram (possibly animated) similar to the Vuex diagram could be very helpful, especially if we can show some code snippets on top of it and visualize how everything connects.
For example, how adding @event-name sets a listener on a child instance that will be triggered when the child calls $emit.

[phanan]

Nice.