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

[Arvind Sudarsanam via llvm-commits]

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@@ -14,10 +14,13 @@ Introduction
 The SPIR-V target provides code generation for the SPIR-V binary format described
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asudarsa wrote:

May be a high-level command that will invoke SPIR-V backend and a list of relevant arguments (with one-line descriptions) will be helpful here?

Thanks

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