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

[James Henderson 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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jh7370 wrote:

How about we rephrase this as: "To mark a test expected failing or unsupported for all cases, use `XFAIL: *` or `UNSUPPORTED: target={{.}}` as appropriate" or something to that effect (most importantly, simply drop the "as a special case" bit)?

I've definitely seen a use-case downstream for marking something as `UNSUPPORTED` for all cases: if a test from upstream has come down and is flaky (so doesn't pass/fail consistently and thus `XFAIL` won't work), it's important to mark it as such whilst it gets reported upstream, so that our continuous merge process doesn't get blocked.

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