count (DISTINCT expression [ , ... ] ) and documentation

I noticed that starting from 8.2 the documentation at
http://www.postgresql.org/docs/8.2/interactive/sql-expressions.html
say that multiple distinct expressions are supported

aggregate_name (DISTINCT expression [, expression] )

While previous docs just listed one:

aggregate_name (DISTINCT expression)

Still I'm using 8.3 and

select count(distinct c1, c2) from table1;

report:

No function matches the given name and argument types. You might
need to add explicit type casts.

What should I write in spite of?

select count(distinct c1, c2) from table1;

--
Ivan Sergio Borgonovo
http://www.webthatworks.it