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/assets/src/lib.rs

@@ -257,6 +257,8 @@ decl_storage! {
 		/// The next asset identifier up for grabs.
 		NextAssetId get(fn next_asset_id): T::AssetId;
 		/// The total unit supply of an asset.
+		///
+		/// TWOX-NOTE: `AssetId` is trusted, so this is safe.
 		TotalSupply: map hasher(twox_64_concat) T::AssetId => T::Balance;
 	}
 }