Fixed configuration for all:propel:param:persistent in database.yml and documentation for database.yml

Sample databases.yml has setting:

persistent: true
So, $options is set to ATTR_PERSISTENT => true
However, in Propel.php  (propel/runtime/lib/Propel.php), the code reads:
private static function processDriverOptions($source, &$write_to)
{
foreach ($source as $option => $optiondata) {
...
...

$value = $optiondata['value'];
...
...
}
}
This sets the $value to null (as $optiondata is true, but
$optiondata['value'] evaluates to null).

So, for it to work, in databases.yml, persistence should be set as:
persistent: {value: true}