Update documentation for toolbox.precache (#234)

Author: mickeywu

docs/api.md

@@ -91,7 +91,7 @@ Like `toolbox.router.get`, etc., but matches any HTTP method.
 Takes a function to use as the request handler for any GET request that does not match a route.
 
 ### `toolbox.precache(arrayOfURLs)`
-Add each URL in arrayOfURLs to the list of resources that should be cached during the service worker install step. Note that this needs to be called before the install event is triggered, so you should do it on the first run of your script.
+Add each URL in arrayOfURLs to the list of resources that should be cached during the service worker install step. URLs in the array can either be a String or Request object. Note that this needs to be called before the install event is triggered, so you should do it on the first run of your script.
 
 ### `toolbox.cache(url, options)`
 Causes the resource at `url` to be added to the cache and returns a Promise that resolves with void. The `options` parameter supports the `debug` and `cache` [global options](#options).