Adjust watch behaviour to match latest RFC update

[trygveaa]

The documentation for watch states:

It also is lazy by default - i.e. the callback is only called when the watched source has changed.

However, when I use it, I see that the callback is called even if the data I watch doesn't change after I called watch. If I set the option { lazy: true }, the callback is not called before the data change.

I also think the documentation should state that this is the same option as immediate in
Vue 2. Unless there are any differences? If so, they should be stated.

[LinusBorg]

The RFC was changed in that regard 10 hours ago, see here.

This plugin has to be updated to match the change in he RFC.