Service worker examples incomplete, do not specify the config object or PUBLIC_URL

[bradjones1]

Describe the bug

The documentation for the service worker example implementation does not explain the config object, referenced at

examples/with-workbox/src/serviceWorkerRegistration.js

Line 23 in bd68019

export function register(config) {

It's kinda-sorta self-explanatory but isn't utilized in the calling code at

examples/with-workbox/App.js

Line 23 in cfb044b

serviceWorkerRegistration.register();

To Reproduce

n/a

Expected behavior

The config object and the required (?) PUBLIC_URL environment variable are documented.