Document vk_icdGetInstanceProcAddr querying device functions

Document vk_icdGetInstanceProcAddr querying device functions

vk_icdGetInstanceProcAddr has similar semantics to vkGetInstanceProcAddr.
It was created to prevent dynamic linker issues. It stated that it should support
global & instance level function querying. But this differs from the Vulkan spec
which states that vkGetInstanceProcAddr should be able to query all functions.
This creates confusion about what takes precendence, the spec or the loader's docs.

This commit makes it explicit that it should have the same semanics what the spec
says.