vre: Acknowledge that limits aren't reentrant

We might want to do something about it once pcre2 is in place, but it
should be noted that concurrent matches on a vre_t could compete over
the limits we can tweak.

Refs #3559

lib/libvarnish/vre.c

@@ -119,6 +119,7 @@ VRE_exec(const vre_t *code, const char *subject, int length,
 	}
 
 	if (lim != NULL) {
+		/* XXX: not reentrant */
 		code->re_extra->match_limit = lim->match;
 		code->re_extra->flags |= PCRE_EXTRA_MATCH_LIMIT;
 		code->re_extra->match_limit_recursion = lim->match_recursion;