Add new guide upload-file

1.  UploadedFile::getInstances($model, 'file'); sees only one file if not set ```'file[]'```

```php
<?= $form->field($model, 'file[]')->fileInput(['multiple' => '']) ?>
```

2. ```$form->errorSummary(..)``` not return all errors of file attribute, but only the last.
So I had to invent.

```php
if ($model->hasErrors()) {
    echo '<pre>';
    print_r($model->getErrors());
    echo '</pre>';
}
```

So it should be?
MS SQL Server fix for older versions than 2012 (alternative solution)

This alternative solution LIMIT, OFFSET problem SQL Server version older than 2012.
fix error

fix error when if fields whose names are reserved word
Update QueryBuilder.php

fix error occurs when you use multiple databases.
fix #3478

it correct work with integer keys of list
Update QueryBuilder.php

solution used **heukirne** for the names of columns.
yii\redis\ActiveRecord::deleteAll() refactoring - no need to write to socket 2 times when nothing found
yii\redis\ActiveRecord::deleteAll() refactoring - no need to write to socket 2 times when nothing found
yii\redis\ActiveRecord::deleteAll() refactoring - code style updated
yii\redis\ActiveRecord::deleteAll() refactoring - code style updated
Merge pull request #3711 from savvot/master

yii\redis\ActiveRecord::deleteAll() refactoring
guide WIP [skip ci]
Fixes #3715: Fixed the bug that using a custom pager/sorter with `GridView` may generate two different pagers/sorters if the layout configures two pagers/sorters
Merge pull request #3479 from lynicidn/patch-4

fix #3478
Issue #3478 added to CHANGELOG
small guide improvements
Merge branch 'master' of github.com:yiisoft/yii2
Fix label() method and make more templates configurable
Merge pull request #3698 from mikehaertl/master

Fix label() method and make more templates configurable
Fixes #3716: `DynamicModel::validateData()` does not call `validate()` if the `$rules` parameter is empty