Curious unnest behavior

I just ran into an interesting thing with unnest and empty arrays.

create table x (
	a int,
	b int[]
);

insert into x(a,b) values (1, '{}');
insert into x(a,b) values (1, '{}');
insert into x(a,b) values (1, '{}');

select a, b from x;
select a, unnest(b) from x;

insert into x(a,b) values (2, '{5,6}');
select a, unnest(b) from x;

drop table x;

gives me:
CREATE TABLE
INSERT 0 1
INSERT 0 1
INSERT 0 1
 a | b  
---+----
 1 | {}
 1 | {}
 1 | {}
(3 rows)

 a | unnest 
---+--------
(0 rows)

INSERT 0 1
 a | unnest 
---+--------
 2 |      5
 2 |      6
(2 rows)

DROP TABLE

I can understand the likely reasoning behind the behavior but perhaps a note in the documentation about it might be of use for others that may get bit by this functionality.  (especially given the structure of the query, had I been doing select * from unnest(arr) that would be more intuitive, but given the query structure of select with no where the results can be surprising.)

thanks

--
Jeff Trout