Improve PQauthDataHook_type docs

Hello,

In https://www.postgresql.org/docs/18/libpq-oauth.html#LIBPQ-OAUTH-AUTHDATA-HOOKS
it is not entirely clear that `hook_fn` is the definition of the
`PQauthDataHook_type` used in the functions signature, because the
name is not used elsewhere. The function signatures refer to the
typedef but the typedef is not shown.

Using https://www.postgresql.org/docs/18/libpq-notice-processing.html#LIBPQ-NOTICE-PROCESSING
as a guideline I suggest to show the typedef of the callback rather
than the `hook_fn` example, so that the `PQauthDataHook_type` name can
be searched for.

Please check the patch attached.

Cheers

-- Daniele