doc (#4761)

substrate/frame/support/src/storage/generator/linked_map.rs

@@ -433,6 +433,8 @@ where
 		}
 	}
 
+	/// The translation happens in-place, new keys are inserted at the same time as old keys are
+	/// removed, thus new keys must not collide with still remaining old keys.
 	fn translate<K2, V2, TK, TV>(translate_key: TK, translate_val: TV) -> Result<(), Option<K2>>
 		where K2: FullCodec + Clone, V2: Decode, TK: Fn(K2) -> K, TV: Fn(V2) -> V
 	{