Added the documentation for multiselect widget

[serhiyzhovnir]

Purpose of this pull request

This pull request (PR) adds documentation about the Multiselect widget.

Affected DevDocs pages

This documentation will have the following links:

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

Page look:

Links to Magento source code

• https://github.com/magento/magento2/blob/2.3/lib/web/mage/multiselect.js#L17
• https://github.com/magento/magento2/blob/2.2/lib/web/mage/multiselect.js#L17
• https://github.com/magento/magento2/blob/2.2/app/code/Magento/Tax/view/adminhtml/templates/rule/edit.phtml#L158
• https://github.com/magento/magento2/blob/2.3/app/code/Magento/Tax/view/adminhtml/templates/rule/edit.phtml#L158

whatsnew
Added a new topic about the Multiselect widget.

[devops-devdocs]

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

[rogyar]

Hi @serhiyzhovnir. As always, thank you for the great topic!

[serhiyzhovnir]

Hi @rogyar
We have improved the explanations of mselectCheckedClass, containerClass, currentPage, lastAppendValue options.
Please, check it again.
Thank you!

[rogyar]

Awesome, thank you

[jeff-matthews]

Thanks for the new topic @serhiyzhovnir! I made a few editorial suggestions.

[serhiyzhovnir]

@jeff-matthews
This PR is adjusted.
Please, let me know if I need to add some additional changes.
Thank you!

[jeff-matthews]

@serhiyzhovnir, I just noticed some markdown syntax errors on your branch that will fail tests. Please fix the following issues so I can finish processing your PR:

rake test:cicd
Testing Markdown style with mdl ...
guides/v2.2/javascript-dev-guide/widgets/widget-multiselect.md:35: MD009 Trailing spaces
guides/v2.3/javascript-dev-guide/widgets/widget-multiselect.md:35: MD009 Trailing spaces

A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

The Markdown linter has found 4 issues

[serhiyzhovnir]

Hi @jeff-matthews
The trailing spaces were removed.
Please, check the PR again.
Thank you!

[jeff-matthews]

Confirmed. Thanks @serhiyzhovnir!

[jeff-matthews]

running tests

[jeff-matthews]

@serhiyzhovnir, tests are failing on broke links:

Running ["LinkChecker::DoubleSlashCheck", "HtmlCheck", "ImageCheck", "LinkCheck", "ScriptCheck"] on ["_site"] on *.html... 


Ran on 2809 files!


- linking to internal hash #updateDelay that does not exist
  *  _site/guides/v2.2/javascript-dev-guide/widgets/widget-multiselect.html (line 275)
     <a href="#updateDelay">updateDelay</a>
  *  _site/guides/v2.3/javascript-dev-guide/widgets/widget-multiselect.html (line 275)
     <a href="#updateDelay">updateDelay</a>
rake aborted!
HTML-Proofer found 2 failures!``` 

[serhiyzhovnir]

Hi @jeff-matthews
The issue with the updateDelay link was fixed.
Thank you!

[jeff-matthews]

running tests

[ghost]

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