update Vue.partial recommendations in migration guide

Author: chrisvfritz

src/guide/migration.md

@@ -1531,7 +1531,11 @@ Use components instead.
 
 ### `Vue.partial` <sup>deprecated</sup>
 
-Use [functional components](render-function.html#Functional-Components) instead.
+Partials have been deprecated in favor of more explicit data flow between components, using props. Unless you're using a partial in a performance-critical area, the recommendation is to simply use a [normal component](components.html) instead. If you were dynamically binding the `name` of a partial, you can use a [dynamic component](http://vuejs.org/guide/components.html#Dynamic-Components).
+
+If you happen to be using partials in a performance-critical part of your app, then you should upgrade to [functional components](render-function.html#Functional-Components). They must be in a plain JavaScript file (rather than in a `.vue` file) and are stateless and instanceless, just like partials. This makes rendering extremely fast.
+
+A benefit of functional components over partials is that they can be much more dynamic, because they grant you access to the full power of JavaScript. There is a cost to this power however. If you've never used a component framework with render functions before, they may take a bit longer to learn.
 
 {% raw %}
 <div class="upgrade-path">