Argument 'week' not supported in date_trunc function with intervals

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/functions-datetime.html
Description:

In the documentation of the date_trunc function (9.9.2), the first argument
(field) cannot be 'week' if the second argument (source) is an interval:

csauer=# SELECT DATE_TRUNC('week', INTERVAL '7 days');


                                                    
ERROR:  interval units "week" not supported because months usually have
fractional weeks
csauer=# SELECT DATE_TRUNC('week', INTERVAL '1 week');


                                                   
ERROR:  interval units "week" not supported because months usually have
fractional weeks

The documentation should state that explicitly.

I also think the error message could be improved, because in the cases above
there are no months involved and the result should be 1.