Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination

fc069a3a63 implements Self-Join Elimination (SJE) and provides a new GUC
variable: enable_self_join_elimination.  This new GUC variable was marked
as GUC_NOT_IN_SAMPLE.  However, enable_self_join_elimination is documented
and is not different from any other enable_* GUCs.  Thus, remove
GUC_NOT_IN_SAMPLE from it and add it to the postgresql.conf.sample.

Discussion: https://postgr.es/m/CAPpHfdsqMTEsmxk3aQwt6xPz%2BKpUELO%3D6fzmER9ZRGrbs4uMfA%40mail.gmail.com
Author: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>

src/backend/utils/misc/guc_tables.c

@@ -998,7 +998,7 @@ struct config_bool ConfigureNamesBool[] =
 		{"enable_self_join_elimination", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables removal of unique self-joins."),
 			NULL,
-			GUC_EXPLAIN | GUC_NOT_IN_SAMPLE
+			GUC_EXPLAIN
 		},
 		&enable_self_join_elimination,
 		true,

src/backend/utils/misc/postgresql.conf.sample

@@ -427,6 +427,7 @@
 #enable_tidscan = on
 #enable_group_by_reordering = on
 #enable_distinct_reordering = on
+#enable_self_join_elimination = on
 
 # - Planner Cost Constants -