Document target_role param of ALTER DEFAULT PRIVILEGES

I just had a confusing moment trying to figure out why my roles
weren't creating tables the way I was expecting them to. I didn't
understand what the `target_role` parameter did. No matter how I try
to read this page, I can't see an explanation for that parameter:

https://www.postgresql.org/docs/current/sql-alterdefaultprivileges.html

I propose an explanation like the following be added to the
`target_role` parameter:

    This is the role that will create objects with altered default
    privileges. Note that changes in default privileges aren't
    inherited, so altering the default privileges of created objects
    for a parent role will not change the default privileges of
    objects created by inherited roles.