Add missing comment in postgresql.conf.

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

Back patched to 9.2 and beyond.

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

@@ -346,6 +346,7 @@
 #syslog_ident = 'postgres'
 
 # This is only relevant when logging to eventlog (win32):
+# (change requires restart)
 #event_source = 'PostgreSQL'
 
 # - When to Log -