Add comments about $watch behavior with a function

[posva]

I realised we are not explicit about this behaviour so I added some comments. I wonder if we should add a full example instead saying it in a comment. We could say that if we got a === 2 and b === 0 and we change both variables to b = 2; a = 0, the watcher won't get triggered

[phanan]

I think with the API doc already saying

Watch an expression or a computed function on the Vue instance for changes. The callback gets called with the new value and the old value. The expression only accepts dot-delimited paths. For more complex expressions, use a function instead.

your added comments should do the job well enough. Taking the liberty to merge now :)