clarify xrEndFrame behavior for overlay sessions.

[yshui]

This line:

OpenXR-Docs/specification/sources/chapters/extensions/extx/extx_overlay.adoc

Lines 194 to 196 in 2b616d9

	If the session is an overlay session, and it provides 0 layers in the call
	to flink:xrEndFrame, then the runtime will just ignore the overlay session
	for the current frame.

is slightly ambiguous. What exactly does "ignore" mean here? Should the runtime retain the most recent image, or should the runtime not show the overlay session for that frame?

[rpavlik-bot]

An issue (number 2156) has been filed to correspond to this issue in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2156 ), to facilitate working group processes.

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