Added Comment Explaining subpiece() Member Function In Range.h
Added Comment Explaining subpiece() Member Function In Range.h

Summary: As titled

Reviewed By: ivmaykov

Differential Revision: D31974990

fbshipit-source-id: 03b60c4e9d4a7464c27cd53f1989cda878132933

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