missing COMMENT ON objects - Mailing list pgsql-hackers

Hi,

I notice you cannot COMMENT ON the following:

* Cast
* Conversion
* Operator class

Docs:
http://developer.postgresql.org/docs/postgres/sql-comment.html

Is that a deliberate omission, or is it an oversight?  If it's 
deliberate, it seems kind of arbitrary.  Or have the docs just not been 
updated?

Chris