Documenting unsupported "required features" in the spec

Some portability implementations cannot support enabling all feature flags that are required for full Vulkan conformance (see Feature Requirements section of spec).

This is a list of features that we might potentially document as not required for portability subset implementations, in the Feature Requirements section of the spec (similar to how we documented robustBufferAccess).

(this list is to be edited)

VkPhysicalDeviceDescriptorIndexingFeatures:

• runtimeDescriptorArray if the implementation cannot support unsized arrays of resources in shaders
• shaderUniformBufferArrayNonUniformIndexing if the implementation cannot support arrays of buffers in shaders (Metal)
• shaderStorageBufferArrayNonUniformIndexing if the implementation cannot support arrays of buffers in shaders (Metal)

Start with the spec's Feature Requirements section and compare its existing treatment of robustBufferAccess with the listed VkPhysicalDeviceDescriptorIndexingFeatures. Determine which listed features should be documented as non-required for portability subset implementations, then update that section so the final exceptions and their conditions are explicit.