[llvm] [doc] Add special case for unsupported test (PR #78858)

[via llvm-commits]

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@@ -568,7 +568,8 @@ list of boolean expressions. The values in each expression may be:
 | ``UNSUPPORTED`` disables the test if any expression is true.
 | ``XFAIL`` expects the test to fail if any expression is true.
 
-As a special case, ``XFAIL: *`` is expected to fail everywhere.
+As a special case, ``XFAIL: *`` is expected to fail everywhere. Similarly,
+``UNSUPPORTED: target={{.*}}`` will disable the test everywhere.
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hiraditya wrote:

> I've definitely seen a use-case downstream for marking something as UNSUPPORTED for all cases

Same. it took me a bit to find this trick with UNSUPPORTED.

https://github.com/llvm/llvm-project/pull/78858