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

That helps disable the test for all the targets

Author: hiraditya

llvm/docs/TestingGuide.rst

@@ -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.
+Use, ``XFAIL: *`` if the test is expected to fail everywhere. Similarly, use
+``UNSUPPORTED: target={{.*}}`` to disable the test everywhere.
 
 .. code-block:: llvm