array_agg() NULL Handling

The aggregate docs say:

> The first form of aggregate expression invokes the aggregate across all input rows for which the given expression(s) yield non-null values. (Actually, it is up to the aggregate function whether to ignore null values or not — but all the standard ones do.)

-- http://developer.postgresql.org/pgdocs/postgres/sql-expressions.html#SYNTAX-AGGREGATES

That, however, is not true of array_agg():

try=# CREATE TABLE foo(id int);
CREATE TABLE
try=# INSERT INTO foo values(1), (2), (NULL), (3);
INSERT 0 4
try=# select array_agg(id) from foo;
array_agg
──────────────
{1,2,NULL,3}
(1 row)

So are the docs right, or is array_agg() right?

Best,

David