Add docs for HTMLDialogElement.uxpShowModal

[yikuansun]

Pretty useful feature IMO

Description

I added documentation for HTMLDialogElement.uxpShowModal in /src/pages/uxp-api/reference-js/Global Members/HTML Elements/HTMLDialogElement.md .

Related Issue

Motivation and Context

uxpShowModal is a great feature that improves the UX of dialogs; however, it is almost impossible to find any info about the method online.

How Has This Been Tested?

I previewed the markdown file in GitHub.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.