Should we document IS [NOT] OF?

Hackers,

Over in news [1] Josh Drake and Eric Ridge discovered the undocumented feature "IS [NOT] OF"; introduced seemingly as an "oh-by-the-way" in 2002 via commit eb121ba2cfe [2].

Is there any reason not to document this back to 9.5, probably with a note nearby to pg_typeof(any), which is a convoluted but documented way of making this kind of test?

David J.

[1] https://www.commandprompt.com/blog/is-of/

[2] https://github.com/postgres/postgres/commit/eb121ba2cfe1dba9463301f612743df9b63e35ce