Document/verify another aspect of OpenACC 'async' semantics in 'libgomp.oacc-c-c++-common/data-3.c'

... that I almost broke with later implementation changes.

	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/data-3.c: Document/verify
	another aspect of OpenACC 'async' semantics.