postgresql.conf.sample: add proper defaults for include actions

Previously, include actions include_dir, include_if_exists, and include
listed commented-out values which were not the defaults, which is
inconsistent with other entries.  Instead, replace them with '', which
is the default value.

Reported-by: Emanuel Araújo

Discussion: https://postgr.es/m/CAMuTAkYMx6Q27wpELDR3_v9aG443y7ZjeXu15_+1nGUjhMWOJA@mail.gmail.com

Backpatch-through: 9.4

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

@@ -739,10 +739,10 @@
 # These options allow settings to be loaded from files other than the
 # default postgresql.conf.
 
-#include_dir = 'conf.d'			# include files ending in '.conf' from
-					# directory 'conf.d'
-#include_if_exists = 'exists.conf'	# include file only if it exists
-#include = 'special.conf'		# include file
+#include_dir = ''			# include files ending in '.conf' from
+					# a directory, e.g., 'conf.d'
+#include_if_exists = ''			# include file only if it exists
+#include = ''				# include file
 
 
 #------------------------------------------------------------------------------