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

[Arvind Sudarsanam via llvm-commits]

================
@@ -62,24 +67,90 @@ to specify the target triple:
      OS                    Description
      ===================== ============================================================
      *<empty>*/``unknown``  Defaults to the OpenCL runtime.
+     ``vulkan``             Vulkan shader runtime.
      ===================== ============================================================
 
   .. table:: SPIR-V Environments
 
      ===================== ==============================================================
      Environment           Description
      ===================== ==============================================================
-     *<empty>*/``unknown``  Defaults to the OpenCL environment.
+     *<empty>*/``unknown``  OpenCL environment or deduced by backend based on the input.
      ===================== ==============================================================
 
 Example:
 
 ``-target spirv64v1.0`` can be used to compile for SPIR-V version 1.0 with 64-bit pointer width.
 
-.. _spirv-types:
+.. _spirv-extensions:
+
+Extensions
+----------
+
+The SPIR-V backend supports a variety of extensions that enable or enhance features beyond the core SPIR-V specification. These extensions can be enabled using the ``-spirv-extensions`` option followed by the name of the extension(s) you wish to enable. Below is a list of supported SPIR-V extensions, sorted alphabetically by their extension names:
----------------
asudarsa wrote:

May be a link to the extension docs will be handy here?
https://github.com/KhronosGroup/SPIRV-Registry/tree/main/extensions

Thanks

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