[DOCS] Clarify docs for AuthorizedEvent

[parkan]

Adding this in reference to the related issue on the non-lambda version of the jwt middleware, hope that's ok!

• Add note about when isPayload is required. The invocation without it will not type check if AuthorizedEvent is parametrized with a TokenPayload, even if that type is empty
• Add APIGatewayEvent intersection type to the handler to clarify how other members on event can be accessed, given that AuthorizedEvent does not extend APIGatewayEvent (ordering APIGatewayEvent before AuthorizedEvent so that any members that override a previous definition would appear as such in the middleware chain -- that doesn't happen now, but it seems like a good practice for e.g. body parser)

Closes #

Checklist:

• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.

[CLAassistant]

All committers have signed the CLA.

[parkan]

fixed a few typos in the above, my keyboard is really about to give up the ghost, thank god I'm not coding in plain js or the typos would eat me alive

[dbartholomae]

Thanks! I did some additional changes (adding the change to the example file which is covered by integration tests to make sure it works and changing commit message/changelog).