Auth0.js: [docs] please document `appState` usage and purpose

[peterblazejewicz]

Description

From time to time the questions about appState opaque local object surface here or on the support forum:
https://community.auth0.com/t/pass-custom-parameters-on-auth0-login-redirect/19075
https://github.com/auth0/auth0.js/issues/662
They are used in the sample code already:
https://auth0.com/docs/quickstart/spa/vuejs#create-an-authentication-service

It could be beneficial to either add the as a note to the docs itself or even change the JDoc to incorporate optional parameter description

Prerequisites

[x] I have checked the documentation for this library.

[x] I have checked the Auth0 Community for related posts.

[x] I have checked for related or duplicate Issues and PRs.

Thanks!

[luisrudge]

I think it's a good idea to add to our jsdocs. Do you want to send a PR with this? 🎉

[peterblazejewicz]

done, thanks!

[luisrudge]

Thanks for the PR!