auth_db: document flag 0 for auth_check function

src/modules/auth_db/doc/auth_db_admin.xml

@@ -549,7 +549,8 @@ if (!proxy_authorize("$fd", "subscriber)) {
 			match the authentication user. All other requests: From header must
 			match the authentication user. If bit 2 is set as well (flags==3),
 			the ID check is skipped for INVITE, BYE, PRACK, UPDATE, MESSAGE
-			- these requests can come with anonymous caller id.
+			- these requests can come with anonymous caller id. If set to 0,
+			then all user checks are skipped.
 			</para>
 			<para>
 			Additionally all domains in the checked URIs and the realm in the