BUG #6178: date_trunc : interval units "week" not supported contradicts documentation

The following bug has been logged online:

Bug reference:      6178
Logged by:          Noah Hamerslough
Email address:      noah@pcc.com
PostgreSQL version: 8.4
Operating system:   Windows Vista
Description:        date_trunc : interval units "week" not supported
contradicts documentation
Details:

http://www.postgresql.org/docs/8.4/static/functions-datetime.html#FUNCTIONS-
DATETIME-TRUNC

The documentation for date_trunc('field', source) lists 'week' in the as a
valid value for 'field' However, if the source is an interval, 'week' is not
supported.

 select date_trunc('week', '1 month 15 days'::interval);

 ERROR: interval units "week" not supported
 SQL state: 0A000

Either 'week' should be supported or the documentation should be updated to
reflect that it is not.