I want a Notice message when save configuration
Above mentioned is correct bit more Explanation on that

Syntax : drupal_set_message($message = NULL, $type = 'status', $repeat = TRUE);

Example usage:

drupal_set_message(t('your message to the user.'), 'error');


Parameters

string $message: (optional) The translated message to be displayed to the user. For consistency with other messages, it should begin with a capital letter and end with a period.

string $type: (optional) The message's type. Defaults to 'status'. These values are supported:

**'status'
'warning'
'error'**


bool $repeat: (optional) If this is FALSE and the message is already set, then the message won't be repeated. Defaults to TRUE.
Return value

array|null A multidimensional array with keys corresponding to the set message types. The indexed array values of each contain the set messages for that type. Or, if there are no messages set, the function returns NULL.