[llvm] [SPIR-V] Extend SPIRVUsage.rst document (PR #84744)

[Arvind Sudarsanam via llvm-commits]

https://github.com/asudarsa approved this pull request.

Looks good overall. It has some very useful pointers for users and developers of the SPIR-V backend. It would be great if this document can be reorganized a bit. But that can happen at a later time.
Added a few minor comments.

Thanks

https://github.com/llvm/llvm-project/pull/84744