Defining and manipulating help text for a path
A quote from the documentation (subheading Parameters):


  $path: The router menu path, as defined in hook_menu(), for the help that is being requested; e.g., 'admin/people' or 'user/register'. If the router path includes a wildcard, then this will appear in $path as %, even if it is a named %autoloader wildcard in the hook_menu() implementation; for example, node pages would have $path equal to 'node/%' or 'node/%/view'. For the help page for the module as a whole, $path will have the value 'admin/help#module_name', where 'module_name" is the machine name of your module.


This means that the following should work for you:

    function mymodule_help($path, $arg) {
      switch ($path) {
        case 'node/YOUR_NODE_ID':
          // Help text for the simple page registered for this path.
          return t('This is help text for the simple page.');

        case 'admin/help#testmodule':
          // Help text for the admin section, using the module name in the path.
          return t("This is help text created in the page example's second case.");
      }
    }