Use more realistic example of custom validation

[begriffs]

Most people will want to use it for logging users out before their JWT has expired. The current example doesn't correspond to any use case.

https://postgrest.com/en/v0.4/auth.html#custom-validation

[steve-chavez]

This section is now at http://postgrest.org/en/v5.2/auth.html#custom-validation.

Not sure for now if this should go in a how-to or a topic guide.

[wolfgangwalther]

We do have some examples of pre-request in the docs now.