config_options.c: Improve misleading warning.

When running "config show help <module>", if no XML documentation exists
for the specified module, "Module <module> not found." is returned,
which is misleading if the module is loaded but simply has no XML
documentation for its config. Improve the message to clarify that the
module may simply have no config documentation.

Resolves: #1489

main/config_options.c

@@ -1239,7 +1239,7 @@ static void cli_show_module_types(struct ast_cli_args *a)
 	ast_assert(a->argc == 4);
 
 	if (!(item = ao2_find(xmldocs, a->argv[3], OBJ_KEY))) {
-		ast_cli(a->fd, "Module %s not found.\n", a->argv[3]);
+		ast_cli(a->fd, "Module %s not found or has no config XML documentation.\n", a->argv[3]);
 		return;
 	}