docs: Simplify auth examples with OUT parameter

The jwt_token type was not created consistently in all examples, which can
be confusing when following those. To return an object with a single key
named token, it's enough to have an OUT parameter to the function.

Resolves https://github.com/PostgREST/postgrest-docs/issues/280

docs/explanations/db_authz.rst

@@ -166,7 +166,7 @@ Another option is to define the function with the :code:`SECURITY DEFINER` optio
   -- login as a user wich has privileges on the private schemas
 
   -- create a sample function
-  create or replace function login(email text, pass text) returns jwt_token as $$
+  create or replace function login(email text, pass text, out token text) as $$
   begin
     -- access to a private schema called 'auth'
     select auth.user_role(email, pass) into _role;