Doc: stop implying recommendation of insecure search_path value.

Doc: stop implying recommendation of insecure search_path value.

SQL "SET search_path = 'pg_catalog, pg_temp'" is silently equivalent to
"SET search_path = pg_temp, pg_catalog, "pg_catalog, pg_temp"" instead
of the intended "SET search_path = pg_catalog, pg_temp".  (The intent
was a two-element search path.  With the single quotes, it instead
specifies one element with a comma and a space in the middle of the
element.)  In addition to the SET statement, this affects SET clauses of
CREATE FUNCTION, ALTER ROLE, and ALTER DATABASE.  It does not affect the
set_config() SQL function.

Though the documentation did not show an insecure command, remove single
quotes that could entice a reader to write an insecure command.
Back-patch to v13 (all supported versions).

Reported-by: Sven Klemm <sven@timescale.com>
Author: Sven Klemm <sven@timescale.com>
Backpatch-through: 13