Tweaked v17 blog post effects changes (#3273)

Author: Brian Vaughn

content/blog/2020-08-10-react-v17-rc.md

@@ -162,17 +162,17 @@ useEffect(() => {
 
 Most effects don't need to delay screen updates, so React runs them asynchronously soon after the update has been reflected on the screen. (For the rare cases where you need an effect to block paint, e.g. to measure and position a tooltip, prefer `useLayoutEffect`.)
 
-However, the effect *cleanup* function, when it exists, used to run synchronously in React 16. We've found that, similar to `componentWillUnmount` being synchronous in classes, this is not ideal for larger apps because it slows down large screen transitions (e.g. switching tabs).
+However, when a component is unmounting, effect *cleanup* functions used to run synchronously (similar to `componentWillUnmount` being synchronous in classes). We've found that this is not ideal for larger apps because it slows down large screen transitions (e.g. switching tabs).
 
-**In React 17, the effect cleanup function also runs asynchronously -- for example, if the component is unmounting, the cleanup will run _after_ the screen has been updated.**
+**In React 17, the effect cleanup function will always runs asynchronously -- for example, if the component is unmounting, the cleanup will run _after_ the screen has been updated.**
 
-This mirrors how the effects themselves run more closely. In the  rare cases where you might want to rely on the synchronous execution, you can switch to `useLayoutEffect` instead.
+This mirrors how the effects themselves run more closely. In the rare cases where you might want to rely on the synchronous execution, you can switch to `useLayoutEffect` instead.
 
 >Note
 >
 >You might be wondering whether this means that you'll now be unable to fix warnings about `setState` on unmounted components. Don't worry -- React specifically checks for this case, and does *not* fire `setState` warnings in the short gap between unmounting and the cleanup. **So code cancelling requests or intervals can almost always stay the same.**
 
-Additionally, React 17 executes the cleanup functions in the same order as the effects, according to their position in the tree. Previously, this order was occasionally different.
+Additionally, React 17 will always execute all effect cleanup functions (for all components) before it runs any new effects. React 16 only guaranteed this ordering for effects within a component.
 
 #### Potential Issues {#potential-issues}