State the purpose of CLEANUP macro

Makefile

@@ -1,4 +1,4 @@
-VERSION   := git-20170213
+VERSION   := git-20170517
 
 PREFIX    := /usr/local
 MANPREFIX := $(PREFIX)/share/man

types.h

@@ -21,6 +21,10 @@
 
 #include <stdbool.h>
 
+/*
+ * Annotation for functions called in cleanup().
+ * These functions are not allowed to call error(!0, ...) or exit().
+ */
 #define CLEANUP
 
 typedef enum {