List installed then available postgresql extensions

gistfile1.sql

postgres=# \dx
                 List of installed extensions
  Name   | Version |   Schema   |         Description          
---------+---------+------------+------------------------------
 plpgsql | 1.0     | pg_catalog | PL/pgSQL procedural language
(1 row)

postgres=# \dx+
      Objects in extension "plpgsql"
            Object Description             
-------------------------------------------
 function plpgsql_call_handler()
 function plpgsql_inline_handler(internal)
 function plpgsql_validator(oid)
 language plpgsql
(4 rows)