kconfig: explain symbol value defaults

Added a few comments - no functional change.
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

scripts/kconfig/expr.h

@@ -65,9 +65,13 @@ enum symbol_type {
 	S_UNKNOWN, S_BOOLEAN, S_TRISTATE, S_INT, S_HEX, S_STRING, S_OTHER
 };
 
+/* enum values are used as index to symbol.def[] */
 enum {
 	S_DEF_USER,		/* main user value */
-	S_DEF_AUTO,
+	S_DEF_AUTO,		/* values read from auto.conf */
+	S_DEF_DEF3,		/* Reserved for UI usage */
+	S_DEF_DEF4,		/* Reserved for UI usage */
+	S_DEF_COUNT
 };
 
 struct symbol {
@@ -75,7 +79,7 @@ struct symbol {
 	char *name;
 	enum symbol_type type;
 	struct symbol_value curr;
-	struct symbol_value def[4];
+	struct symbol_value def[S_DEF_COUNT];
 	tristate visible;
 	int flags;
 	struct property *prop;