Generates a string representation of an array of 'allowed values'.
This string format is suitable for edition in a textarea.
$values: An array of values, where array keys are values and array values are labels.
The string representation of the $values array:
function list_allowed_values_string($values) {
$lines = array();
foreach ($values as $key => $value) {
$lines[] = "{$key}|{$value}";
}
return implode("\n", $lines);
}