Descriptor Heap: Add small Shader Record section

[spencer-lunarg]

There is a TODO at https://github.com/KhronosGroup/Vulkan-Guide/blob/main/chapters/descriptor_heap.adoc#vkdescriptormappingsourceext-shader-record

Basically I think all we need is a small thing to say

1. Shader Records are a Ray Tracing concept (for those who don't know) and maybe link to where they are better described/defined (I don't think we have any information in the Vulkan Guide chapter, so maybe a spec link)

2. SHADER_RECORD is just (double check my math here please)

• VK_DESCRIPTOR_MAPPING_SOURCE_HEAP_WITH_SHADER_RECORD_INDEX_EXT == VK_DESCRIPTOR_MAPPING_SOURCE_HEAP_WITH_INDIRECT_INDEX_EXT
• VK_DESCRIPTOR_MAPPING_SOURCE_SHADER_RECORD_DATA_EXT == VK_DESCRIPTOR_MAPPING_SOURCE_RESOURCE_HEAP_DATA_EXT
• VK_DESCRIPTOR_MAPPING_SOURCE_SHADER_RECORD_ADDRESS_EXT == VK_DESCRIPTOR_MAPPING_SOURCE_PUSH_ADDRESS_EXT

but for a shader record

3. Anything else not obvious worth putting in her