Add comments about definitions that may affect PG_CONTROL_VERSION,

per recent unintended-initdb-forcing fiasco

src/include/catalog/pg_control.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/pg_control.h,v 1.56 2010/06/03 14:50:30 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/pg_control.h,v 1.57 2010/06/03 20:37:13 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,6 +26,7 @@
 /*
  * Body of CheckPoint XLOG records.  This is declared here because we keep
  * a copy of the latest one in pg_control for possible disaster recovery.
+ * Changing this struct requires a PG_CONTROL_VERSION bump.
  */
 typedef struct CheckPoint
 {
@@ -60,7 +61,10 @@ typedef struct CheckPoint
 #define XLOG_PARAMETER_CHANGE			0x60
 
 
-/* System status indicator */
+/*
+ * System status indicator.  Note this is stored in pg_control; if you change
+ * it, you must bump PG_CONTROL_VERSION
+ */
 typedef enum DBState
 {
 	DB_STARTUP = 0,