Update FEATURE_VULKAN_HARDWARE_COMPUTE documentation

Now that VK_KHR_variable_pointers is public, the documentation can be more
precise about the definition of FEATURE_VULKAN_HARDWARE_COMPUTE level 0.
Also document the maxPerStageDescriptorStorageBuffers requirement.

Bug: 34745152
Test: make docs
Change-Id: I0f6c088488243e4daea589d4771d10a8714f7441