doc: Clarify requirements for SET ROLE.

Since commit 3d14e17, SET ROLE has required the current session
user to have membership with the SET option in the target role, but
the SET ROLE documentation only mentions the membership
requirement.  This commit adds this important detail to the SET
ROLE page.

Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/CA%2BRLCQysHtME0znk2KUMJN343ksboSRQSU-hCnOjesX6VK300Q%40mail.gmail.com
Backpatch-through: 16

Author: nathan-bossart

doc/src/sgml/ref/set_role.sgml

@@ -41,8 +41,10 @@ RESET ROLE
   </para>
 
   <para>
-   The specified <replaceable class="parameter">role_name</replaceable>
-   must be a role that the current session user is a member of.
+   The current session user must have the <literal>SET</literal> option for the
+   specified <replaceable class="parameter">role_name</replaceable>, either
+   directly or indirectly via a chain of memberships with the
+   <literal>SET</literal> option.
    (If the session user is a superuser, any role can be selected.)
   </para>