Improve docs on the session prop

[sodic]

Wasp automatically passes the user prop to authenticated components (i.e., pages with auth: true).

We show this functionality in the todoApp tutorial, but our documentation only mentions the useAuth hook (which is a suboptimal way of getting the user because the hook can't know whether it's being called in an authenticated component).

Our documentation should clearly instruct our users to use the user prop whenever possible and only resort to the useAuth hook when they want to do something user-related in a non-authenticated component (e.g., headers).

Pay attention to our writing guide: https://github.com/wasp-lang/wasp/blob/main/web/docs/writingguide.md

[FranjoMindek]

Changed with #2535