My expectation was to create a tab in the dialog where user can select where the description has to display .. like completely left , middle and right
There is no limitation for that, you can use richtext field where ever you want.
Please check below example where you can find dialog structure for richtext which is included at tab level:
https://gist.github.com/briankasingli/1a7e3e12deaa2e076645b09d30039b46