Add notes about safe uses of twox (#6082)

* 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>

substrate/frame/babe/src/lib.rs

@@ -152,6 +152,8 @@ decl_storage! {
 		/// We reset all segments and return to `0` at the beginning of every
 		/// epoch.
 		SegmentIndex build(|_| 0): u32;
+
+		/// TWOX-NOTE: `SegmentIndex` is an increasing integer, so this is okay.
 		UnderConstruction: map hasher(twox_64_concat) u32 => Vec<schnorrkel::Randomness>;
 
 		/// Temporary value (cleared at block finalization) which is `Some`