OpenACC 'kernels' decomposition: wrong-code cases unless manually making certain...

OpenACC 'kernels' decomposition: wrong-code cases unless manually making certain variables addressable [PR104892]

Document a few examples of the status quo.

	PR middle-end/104892
	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/kernels-decompose-1.c: Point
	to PR104892.
	* testsuite/libgomp.oacc-c-c++-common/default-1.c: Likewise,
	enable '--param=openacc-kernels=decompose' and adjust.
	* testsuite/libgomp.oacc-c-c++-common/kernels-reduction-1.c:
	Likewise.
	* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c: Likewise.
	* testsuite/libgomp.oacc-fortran/kernels-reduction-1.f90:
	Likewise.