Clarify xrEnumerateInstanceExtensionProperties

[fredemmott]

Per https://registry.khronos.org/OpenXR/specs/1.1/html/xrspec.html#api-initialization , this function can be called before calling xrCreateInstance.

This function may: be called before an instance has been created; implementations must: not assume an instance exists.

Concretely:

• a bug in past versions of the Ultraleap API layer crashed if an instance had not been created.
• I've added the paragraph explaining about API layers invoking these, as a past version of the Ultraleap API layer entirely removed their implementation of this and xrEnumerateApiLayerProperties as they appeared to be dead code

[fredemmott]

Related loader docs PR: https://github.com/KhronosGroup/OpenXR-SDK-Source/pull/490

[rpavlik-bot]

An issue (number 2322) has been filed to correspond to this pull request in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2322 ), to facilitate working group processes.

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