text: Title: Docs: Add guard example to JWT docs
  Body: ### Summary
  
  Currently in the [JWT docs](https://docs.litestar.dev/latest/usage/security/jwt.html) there are a few references regarding how to access the 'User' object during and after being authenticated. These boil down to:
  
  a) queried using the token
  b) directly from the request (which can be easily assumed to be attached via retrieve_user_handler prior to going to the api path)
  
  However there are other instances where user details need to be extracted from the `connection` object (such as in role-based guards).
  ```py
  def admin_guard(connection: ASGIConnection, _: BaseRouteHandler) -> None:
      if not connection.user.is_admin:
          raies NotAuthorizedException()
  ```
  
  A gap in knowledge between the page on JWTs and guards is that it's not made entirely clear _how_  user gets attached to connection. I would like to suggest that an example guard is added to the JWT docs with a comment explaining that the Auth object automatically attaches it for you based on the object returned from `retrieve_user_handler`. 
  
  It also isn't made abundantly clear that the TypeVar provided to the Auth object directly corresponds to the retrieve_user_handler. For a little while, I was actually setting the TypeVar based on my login response and wondering why it wasn't working. A silly mistake in hindsight, but I believe a simple comment could have saved me from it!