Rename session_auth_is_superuser to current_role_is_superuser.
This variable might've been accurately named when it was added in
ea886339b8, but the name hasn't been accurate since at least the
introduction of SET ROLE in
e5d6b91220. The corresponding
documentation was fixed in
eedb068c0a. This commit renames the
variable accordingly.
Suggested-by: Joseph Koshakow
Discussion: https://postgr.es/m/CAAvxfHc-HHzONQ2oXdvhFF9ayRnidPwK%2BfVBhRzaBWYYLVQL-g%40mail.gmail.com