add documentation for authenticate()

inc/inc.ClassAuthenticationService.php

@@ -63,6 +63,19 @@ class SeedDMS_AuthenticationService {
 		return $this->errors;
 	} /* }}} */
 
+	/**
+	 * Run each authentication service
+	 *
+	 * This method calls authenticate() of each authentication service and
+	 * evaluates the returned value.
+	 * If the authentication method returns false (some internal error which
+	 * prevented to check authentication at all), this method will return
+	 * false imediately, in case of null (no valid authentication) the next
+	 * service will be tried and in all other cases the value will be returned.
+	 *
+	 * @param string $username name of user
+	 * @param string $password password of user
+	 */
 	public function authenticate($username, $password) { /* {{{ */
 		$user = null;
 		foreach($this->services as $name => $service) {