loader: Clarify in docs about WSI and inst ext.
loader: Clarify in docs about WSI and inst ext.

Clarify in the LoaderAndLayerInterface markdown file what WSI
extensions are available by default in the desktop loader.
Additionally, clarify that instance extensions are not supported
by the Loader unless they have been specifically added to the
Loader.

Change-Id: I21bd2a63ed1e4a7e50d27e7a6b136eeeaea4d68c

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools