"20.16. Customized Options" – cannot be set by `ALTER SYSTEM`

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/runtime-config-custom.html
Description:

As far as I can tell, the following statement:

> PostgreSQL will accept a setting for any two-part parameter name

does not hold when creating a *new* setting with `ALTER SYSTEM`, e.g.

ALTER SYSTEM SET foo.bar TO 'baz';

will elicit an error.

However, if `foo.bar` is defined in `postgresql.conf` or
`postgresql.auto.conf` – put there by hand – then it can be altered, i.e.
the `ALTER SYSTEM` command above will succeed.

I don't know if this is something that should be mentioned in the
documentation or if it's an inconsistency in the implementation.