Clarify `XrCompositionLayerPassthroughHTC::layerFlags`

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.

Start with the linked OpenXR specification section for XrCompositionLayerPassthroughHTC::layerFlags and compare its requirement with the other composition-layer definitions. Determine why zero flags are disallowed, then clarify the specification or document the intended alternative; done means the restriction has an explicit rationale or is resolved.

Written by the indexing model from the issue text.