Provide plan_cache_mode options in postgresql.conf.sample.

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f8YkwojSTSg8YjNYCLCXzx0fR7wBR3Gf%2BrA9_52eoPZKg%40mail.gmail.com

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

@@ -350,7 +350,7 @@
 #join_collapse_limit = 8		# 1 disables collapsing of explicit
 					# JOIN clauses
 #force_parallel_mode = off
-#plan_cache_mode = auto
+#plan_cache_mode = auto			# auto, force_generic_plan or force_custom_plan
 
 
 #------------------------------------------------------------------------------