Clarify `XrCompositionLayerPassthroughHTC::layerFlags`

[yshui]

Spec:

https://registry.khronos.org/OpenXR/specs/1.0/html/xrspec.html#XrCompositionLayerPassthroughHTC

layerFlags must not be 0

But it never explains why the author thinks it is impossible for no flags to apply. And this limitation doesn't appear for other composition layers.

I can always set CORRECT_CHROMATIC_ABERRATION because it's essentially no-op, but that's said to be on deprecation path.

[rpavlik-bot]

An issue (number 2503) has been filed to correspond to this issue in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2503 ), to facilitate working group processes.

This GitHub issue will continue to be the main site of discussion.