Unify globals examples in docs

[schmunk42]

What steps will reproduce the problem?

Read the docs.

What's expected?

Identical keys for identical classes (same as class name).

'Url' => ['class' => '\yii\helpers\Url'],

What do you get instead?

'Url' => ['class' => '\yii\helpers\Url'],

• https://github.com/yiisoft/yii2-twig/blob/master/docs/guide/template-syntax.md#importing-other-classes

---

'url' => ['class' => '\yii\helpers\Url'], // new global

• https://github.com/yiisoft/yii2-twig/blob/master/docs/guide/layouts-and-widgets.md#navigation-bar

Additional info

Q	A
Yii vesion	n/a
PHP version	n/a
Operating system	n/a

The lowercase url() built-in helper function could also be differentiated better if using CamelCase keys. Changing this later in an application is quite some pain, since it is case-sensitive.