doc: Note exceptions for SET ROLE's effect on privilege checks.

doc: Note exceptions for SET ROLE's effect on privilege checks.

The documentation for SET ROLE states that superusers who switch to
a non-superuser role lose their superuser privileges.  While this
is true for most commands, there are exceptions such as SET ROLE
and SET SESSION AUTHORIZATION, which continue to use the current
session user and the authenticated user, respectively.
Furthermore, the description of this command already describes its
effect, so it is arguably unnecessary to include this special case.
This commit removes the note about the superuser case and adds a
sentence about the aforementioned exceptions to the description.

Co-authored-by: Yurii Rashkovskii
Reviewed-by: Shubham Khanna, Robert Haas, Michael Paquier
Discussion: https://postgr.es/m/CA%2BRLCQysHtME0znk2KUMJN343ksboSRQSU-hCnOjesX6VK300Q%40mail.gmail.com