Update taskstats-struct document for scaled time accounting

Update Documentation/accounting/taskstats-struct.txt for TASKSTATS_VERSION 6,
adding scaled time accounting.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>