doc: Add note about "Client User" and "Superuser" fields in \conninfo output.

doc: Add note about "Client User" and "Superuser" fields in \conninfo output.

In the \conninfo psql command, the "Client User" column shows the user who
established the connection, while the "Superuser" column reflects whether
the current user in the current execution context is a superuser. This means
the users referred to in these columns can differ, for example, if the current
user was changed with the SET ROLE command.

This commit adds a note to the \conninfo documentation to clarify
this behavior and avoid potential confusion.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/685961b8-b6ce-40bb-b2d5-c2ff135d3388@oss.nttdata.com