Apply suggestions from code review

Co-authored-by: Andrew Betts <[email protected]>

Author: JakeChampion

documentation/docs/globals/FetchEvent/FetchEvent.mdx

@@ -12,7 +12,7 @@ It provides the [`event.respondWith()`](./prototype/respondWith.mdx) method, whi
 ## Instance properties
 
 - `FetchEvent.request` _**readonly**_
-    - : The `Request` that was recieved by the application.
+    - : The `Request` that was received by the application.
 - `FetchEvent.client` _**readonly**_
     - : Information about the downstream client that made the request.
     - `FetchEvent.client.address` _**readonly**_

documentation/docs/globals/FetchEvent/prototype/respondWith.mdx

@@ -22,4 +22,4 @@ respondWith(response)
 
 ### Return value
 
-None `undefined`.
+Always returns `undefined`.

documentation/docs/globals/FetchEvent/prototype/waitUntil.mdx

@@ -10,8 +10,8 @@ pagination_prev: null
 The **`waitUntil()`** method tells the host environment that work is ongoing until the promise settles, and it shouldn't terminate
 the application if it wants that work to complete.
 
-The `waitUntil()` method must be initially called within the event callback,
-but after that it can be called multiple times, until all the promises passed to it
+The `waitUntil()` method must be initially called synchronously within the event callback,
+but after that it can be called multiple times, and will hold the process open until all the promises passed to it
 settle.
 
 ## Syntax