Clearly state in the guide which data sources trigger re-evaluation of a computed property

[LinusBorg]

vuejs/vue#3517

E.g. this.$slots is not reactive, so a change in slots would not trigger a re-evaluation

[FEA5T]

Would be cool if there was a reactivity table on this. Could save a lot of writing/explaining.

Source vs Dest with each cell reactive or not.

Sources could be: data, methods, browser api's, computed's, props, injects, etc
Dest would be: ^ those + dom - browser api's

[mgrollins]

With the recent addition of the Vue.observable(object) api_doc the main guide needs updating, especially around line numbers 63, 69, and 81 in instance.md page.

I might be able to update this documentation, but I'm still fairly new to the ecosystem and especially JS build systems. I'm going to be attempting to add some reactivity to an object when using Vue through the method of a direct <script> include, and might learn enough from the process to contribute something back. If so I will fork and submit (my 1st gh) PR!