[Documentation]: Dialog documentation lacks information about how to customize Portal mount node

[sunsin10]

Library

@fluentui/react-components (v9 dialog)

Describe the feature that you would like added

Dialog documentation should have one section where it will explain the tips to customize the z-index of dialog, I had issue where existing dialog has same z-index which dialog also had the same. Using mountNode class style helped me to set the z-index, Also is there any usage of Portal component in dialog component?

Have you discussed this feature with our team

No response

Additional context

No response

Validations

• [X] Check that there isn't already an issue that requests the same feature to avoid creating a duplicate.

Priority

Low

[layershifter]

We should not have component specific docs for this (as other components also use mountNode prop). Instead we should have shared docs with examples like for positioning and improve its visibility (#25098).