Shift+Meta+Space should reverse-rotate subtypes part 2.

This CL adds JavaDoc to clarify what previous CL [1] wanted to do.
No behavior change is intended in this CL.

 [1]: I3694edd80be6dfe18b90360e24ae4d451b331928
      d39ae854

Bug: 25753404
Bug: 28103839
Change-Id: I246223c0856382d68323f22987b998cd1613e98c