Document values of stats_fetch_consistency in postgresql.conf.sample

Issue noted while looking at a patch related to that.

Discussion: https://postgr.es/m/ZE9LiFc7JdNHokz/@paquier.xyz

Author: michaelpq

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

@@ -621,7 +621,7 @@
 #track_io_timing = off
 #track_wal_io_timing = off
 #track_functions = none			# none, pl, all
-#stats_fetch_consistency = cache
+#stats_fetch_consistency = cache	# cache, none, snapshot
 
 
 # - Monitoring -