Add commenents to explain the Vulkan ASTC situation.

Expose GL_KHR_texture_compression_astc_sliced_3d when the HDR profile
exists.

Bug: angleproject:4447
Change-Id: I00a86cb3c44873a28397aab33a47fb363d4ecfe9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2112283Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Alexey Knyazev <lexa.knyazev@gmail.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>