Add code samples for alert widget and add information about modalClass option

[serhiyzhovnir]

Purpose of this pull request

This pull request (PR) adds code samples of the alert widget initialization.
Also, it adds information about the modalClass widget option.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.3/javascript-dev-guide/widgets/widget_alert.html
• https://devdocs.magento.com/guides/v2.2/javascript-dev-guide/widgets/widget_alert.html
• https://devdocs.magento.com/guides/v2.1/javascript-dev-guide/widgets/widget_alert.html

Links to Magento source code

• https://github.com/magento/magento2/blob/2.1/app/code/Magento/Ui/view/base/web/js/modal/alert.js#L17
• https://github.com/magento/magento2/blob/2.2/app/code/Magento/Ui/view/base/web/js/modal/alert.js#L20
• https://github.com/magento/magento2/blob/2.3/app/code/Magento/Ui/view/base/web/js/modal/alert.js#L20
• https://github.com/magento/magento2/blob/2.1/app/code/Magento/Sales/view/adminhtml/web/order/create/scripts.js#L1291
• https://github.com/magento/magento2/blob/2.2/app/code/Magento/Sales/view/adminhtml/web/order/create/scripts.js#L1090
• https://github.com/magento/magento2/blob/2.3/app/code/Magento/Sales/view/adminhtml/web/order/create/scripts.js#L1473

[devops-devdocs]

An admin must run tests on this PR before it can be merged.

[serhiyzhovnir]

Hi @erikmarr
The required changes were added.
Can you, please, check it again?
Thank you!

[erikmarr]

running tests

[ghost]

Hi @serhiyzhovnir, thank you for your contribution!
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[erikmarr]

Thanks @serhiyzhovnir!