Clarify the use of `XrPassthroughHTC` handles

Two questions I have:

1. can multiple `XrPassthroughHTC` handles created from a single `XrSession` exist at the same time?
2. can multiple `XrCompositionLayerPassthroughHTC` layers referencing the same `XrPassthroughHTC` handle exist in the same `xrEndFrame` call? if so is there a limit to how many such layers there can be?

The spec says (emphasis mine):

> An application can create *an* `XrPassthroughHTC` handle by calling `xrCreatePassthroughHTC`.

If I take this "an" as face value, this seems to state only one such handle may be created? similarly:

> The application can create *an* `XrCompositionLayerPassthroughHTC` structure with the created passthrough and the corresponding information. *A* pointer to `XrCompositionLayerPassthroughHTC` may be submitted in `xrEndFrame`

Is that the correct reading?

Start with the specification text for xrCreatePassthroughHTC, XrPassthroughHTC, XrCompositionLayerPassthroughHTC, and xrEndFrame. Determine whether the wording defines limits on handles or submitted layers, then update the relevant specification language so both questions have explicit answers.

Written by the indexing model from the issue text.