Added docs about TTL index for the `Session`
Added docs about TTL index for the `Session`

MysteryDragon/yii2-mongodb,klimov-paul/yii2-mongodb,yiijan/yii2-mongodb,MysteryDragon/yii2-mongodb,yiijan/yii2-mongodb,klimov-paul/yii2-mongodb,yiisoft/yii2-mongodb