document that PF_CFG_BLOCK_FINAL is internal/experimental

src/vdp_pesi.c

@@ -415,7 +415,8 @@ vped_include(struct req *preq, const char *src, const char *host,
 
 	if (pesi->flags & PF_CFG_BLOCK_FINAL) {
 		/* XXX because of T_FINAL blocking in the subreq thread, these
-		 * threads *have* to run.
+		 * threads *have* to run - which is why PF_CFG_BLOCK_FINAL is
+		 * not available from VCL
 		 *
 		 * ordering should ensure that we run top->bottom left-right in
 		 * the ESI tree, so delivery should be able to let T_FINAL