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

[Saleem Abdulrasool 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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compnerd wrote:

@jh7370 I agree with you, but I guess I didn't read it as "this is also a special case", but rather as "this is another way to do this". I think that having the pointer to the fact that this is not great is important since we don't want to encourage people to disable tests I think.

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