One useful feature of SQL Server is the ability to use Extended Properties to add descriptions for tables and columns. I find it helpful to be able to document databases like this, to have these descriptions as part of the database.
Luckily, Postgres has a similar feature, adding comments to tables and columns.

Add Comment:

COMMENT ON TABLE public.test_table IS 'Table Comment';

COMMENT ON COLUMN public.test_table.test_table_id 
    IS 'Column Comment';

Retrieve Comments:

Here are some queries to retrieve comments for tables and for columns.
information_schema.tables will return tables and views, so I’ve added the table_type = ‘BASE TABLE’ clause to limit the results to tables.

SELECT table_schema, table_name, 
    OBJ_DESCRIPTION(CONCAT(table_schema, '.', 
    table_name)::regclass) as description
FROM information_schema.tables
WHERE table_catalog = 'test_db'
    AND table_schema = 'public'
    AND table_type = 'BASE TABLE'
ORDER BY table_name;

SELECT c.table_schema, c.table_name, c.column_name,
    COL_DESCRIPTION(CONCAT(c.table_schema, '.', 
    c.table_name)::regclass, ordinal_position) as description
FROM information_schema.columns as c
JOIN information_schema.tables as t
    ON t.table_catalog = c.table_catalog
    AND t.table_schema = c.table_schema
    AND t.table_name = c.table_name
WHERE c.table_catalog = 'test_db'
    AND c.table_schema = 'public'
    AND t.table_type = 'BASE TABLE'
ORDER BY c.table_name, c.ordinal_position;

The information_schema views return other information, like the data type and nullability for columns, so you would have enough information available to generate a data dictionary for the database.