Seems there are three GUC variables that are defined as "Shows ..."

while you can actually set them with SET.

This applied patch changes the wording from "Show" to "Set".

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.172 2003/11/29 19:52:03 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.173 2003/12/01 03:55:21 momjian Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -808,7 +808,7 @@ static struct config_bool ConfigureNamesBool[] =
 	},
 	{
 		{"transaction_read_only", PGC_USERSET, CLIENT_CONN_STATEMENT,
-			gettext_noop("Shows the current transaction's read-only status."),
+			gettext_noop("Sets the current transaction's read-only status."),
 			NULL,
 			GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
@@ -1532,7 +1532,7 @@ static struct config_string ConfigureNamesString[] =
 	{
 		/* Not for general use --- used by SET SESSION AUTHORIZATION */
 		{"session_authorization", PGC_USERSET, UNGROUPED,
-			gettext_noop("Shows the session user name."),
+			gettext_noop("Sets the session user name."),
 			NULL,
 			GUC_REPORT | GUC_NO_SHOW_ALL | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
@@ -1572,7 +1572,7 @@ static struct config_string ConfigureNamesString[] =
 
 	{
 		{"transaction_isolation", PGC_USERSET, CLIENT_CONN_STATEMENT,
-			gettext_noop("Shows the current transaction's isolation level."),
+			gettext_noop("Sets the current transaction's isolation level."),
 			NULL,
 			GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},