added some comments and dokus

html/checknewuser.php

@@ -1,4 +1,20 @@
 <?php
+/*! @file
+
+    API-call checknewuser.php
+
+    Check if a user exists in the server's user table.
+
+    @param user user name to check
+    @return json encoded value:
+      - 'user name as string', if user does exist
+      - null, if user does not exist or in case of error
+
+    @id $Id$
+*/
+//       1         2         3         4         5         6         7         8
+// 45678901234567890123456789012345678901234567890123456789012345678901234567890
+
 require_once("usertable.php");
 try {
   $user = $db->real_escape_string($_REQUEST['user']);