Improved backplate edge smoothing docs

[Cameron-Micka]

Describe the issue

Need to document when and why to enable edge smoothing for backplates.

Feature area

Missing nuances between when and how edge smoothing works for backplates on various endpoints.

Existing doc link

https://docs.microsoft.com/en-us/windows/mixed-reality/mrtk-unity/mrtk3-graphicstools/architecture/mrdl-mrgt#back-plate

Additional context

When smooth edges are enabled, the backplate should be alpha blended unless the display treats black as transparent.