Clarify a fine point in the VAV api.

lib/libvarnish/vav.c

@@ -31,6 +31,8 @@
  * const char **VAV_Parse(const char *s, int *argc, int flag)
  *	Parse a command like line into an argv[]
  *	Index zero contains NULL or an error message
+ *	(The error message is a static const char* and does not
+ * 	need saving or copying.)
  *	"double quotes" and backslash substitution is handled.
  *
  * void VAV_Free(const char **argv)