Add missing comment in postgresql.conf.

dynamic_shared_memory_type requires to restart server to reflect
the new value. Per Yugo Nagata and Masahiko Sawada.

Back pached to 9.4 and beyond.

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

@@ -131,6 +131,7 @@
 					#   windows
 					#   mmap
 					# use none to disable dynamic shared memory
+					# (change requires restart)
 
 # - Disk -