Revise beforeUpdate API entry, fixes vuejs/vue#7481 (#1395)

Author: chrisvfritz

src/v2/api/index.md

@@ -794,11 +794,9 @@ type: api
 
 - **Details:**
 
-  Called when the data changes, before the virtual DOM is re-rendered and patched.
+  Called when data changes, before the DOM is patched. This is a good place to access the existing DOM before an update, e.g. to remove manually added event listeners.
 
-  You can perform further state changes in this hook and they will not trigger additional re-renders.
-
-  **This hook is not called during server-side rendering.**
+  **This hook is not called during server-side rendering, because only the initial render is performed server-side.**
 
 - **See also:** [Lifecycle Diagram](../guide/instance.html#Lifecycle-Diagram)