* Add notes about safe uses of twox

* Update frame/grandpa/src/lib.rs

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

* Update frame/elections/src/lib.rs

* Apply suggestions from code review

Co-authored-by: Gavin Wood <gavin@parity.io>
Co-authored-by: Nikolay Volf <nikvolf@gmail.com>