clarify .sync modifier usage

Author: chrisvfritz

src/v2/guide/components-custom-events.md

@@ -155,4 +155,6 @@ The `.sync` modifier can also be used with `v-bind` when using an object to set
 <text-document v-bind.sync="doc"></text-document>
 ```
 
-This has the effect of adding `v-on` update listeners for not only `title`, but also any other properties on the `doc` object.
+This passes each property in the `doc` object (e.g. `title`) as an individual prop, then adds `v-on` update listeners for each one.
+
+<p class="tip">Using <code>v-bind.sync</code> with a literal object, such as in <code>v-bind.sync="{ title: doc.title }"</code>, will not work. If you want to include multiple, unrelated data properties in the same <code>v-bind.sync</code>, we recommend creating a computed property that returns an object.</p>