Is there a postgres function documentation pattern that is similar in concept to javadoc?
Not really, however in Postgres and many other DBMS vendors you can comment on various database objects like:

COMMENT ON FUNCTION my_function (timestamp) IS 'Returns Roman Numeral';


As far as I can tell there is no way you can comment on individual arguments for a function. 

For the presentation of the documentation, you may want to have a look at:

http://schemaspy.org/