align_metadata_size documents metadata alignment

align_metadata_size(x) returns align_up(x, 1), which is a complicated (and probably constant-folded) way to write 'x'.

JDK-8145628 introduced align_metadata_size with that definition. Since the size is argument in supposed to be in words, it seems correct. But the definition could be simplified. The assumption that the size is words should also be mentioned. Or maybe it should be moved elsewhere and changed to use the WordSize class