Add code samples for confirm widget and added information about modalClass option

[serhiyzhovnir]

Purpose of this pull request

This pull request (PR) adds code samples for the confirm widget and also it adds the information about modalClass option.

Affected DevDocs pages

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

Links to Magento source code

• https://github.com/magento/magento2/blob/2.1/app/code/Magento/Ui/view/base/web/js/modal/confirm.js#L17
• https://github.com/magento/magento2/blob/2.2/app/code/Magento/Ui/view/base/web/js/modal/confirm.js#L20
• https://github.com/magento/magento2/blob/2.3/app/code/Magento/Ui/view/base/web/js/modal/confirm.js#L20
• https://github.com/magento/magento2/blob/2.3/app/code/Magento/Checkout/view/frontend/web/js/sidebar.js#L90
• https://github.com/magento/magento2/blob/2.2/app/code/Magento/Checkout/view/frontend/web/js/sidebar.js#L90
• https://github.com/magento/magento2/blob/2.1/app/code/Magento/Checkout/view/frontend/web/js/sidebar.js#L74

[devops-devdocs]

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

[dobooth]

Hi @serhiyzhovnir Thanks for this. My comments about the images in #4731 also apply here.

[serhiyzhovnir]

Hi @dobooth
We have created new PR with requested changes: #4781
Thank you!

[ghost]

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