Modify comments in Future.h to use thenValue, thenTry and thenError rather than then and onError
Modify comments in Future.h to use thenValue, thenTry and thenError rather than then and onError

Summary: Make comments consistent with the updated recommendations where it makes sense.

Reviewed By: yfeldblum

Differential Revision: D9833280

fbshipit-source-id: 8de3372b013a322e57cd2d54a512d75bf963d1a5

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly