`IndexMap`s with different orders but the same key are equal

[Luro02]

This is because IndexMap is an alias of ArrayMapFacade. The indexmap's IndexMap has the same implementation, but this is something that should be documented.

https://github.com/bluss/indexmap/blob/35b36eb83338d7c8d3f2a60013e5f5ac020c2c71/src/map.rs#L1400-L1415