gigasecond: add clarity to exercise goal (#718)
gigasecond: add clarity to exercise goal (#718)

This helps address my comments in
https://github.com/exercism/rust/pull/710#issuecomment-438078198

https://github.com/exercism/problem-specifications/pull/1398
https://github.com/exercism/problem-specifications/pull/1404
exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust