[Documentation] Update for watchers

[bert-w]

I found out via StackOverflow that you can watch a nested object property like this:

watch: {
     'someObject.someProp': function (newVal, oldVal) {
        // some logic
     },

This would then trigger when changes are made to this property.

I wasnt able to find this in the documentation. Is it defined somewhere, and if not where's the best page to add it to? https://vuejs.org/v2/guide/computed.html#Watchers ?

[bencodezen]

Thanks for the suggestions @bert-w! I'll label this as post-3.0 so we can make sure our examples are a bit more comprehensive for watchers.