[llvm] [Offload] Add Offload API Sphinx documentation (PR #147323)

[Shilei Tian via llvm-commits]

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@@ -44,6 +45,8 @@ cl::opt<ActionType> Action(
         clEnumValN(DumpJSON, "dump-json",
                    "Dump all records as machine-readable JSON"),
         clEnumValN(GenAPI, "gen-api", "Generate Offload API header contents"),
+        clEnumValN(GenSpec, "gen-spec",
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shiltian wrote:

Do we want to call it a "spec" here? Is it a spec or a document? I think they are different.

https://github.com/llvm/llvm-project/pull/147323