sentinel-doc.patch documents the contrary behaviour -- not preserving an identity and equality.

I haven't included doc changes because I don't know how to treat this change. As a bug fix, as a new feature, or as a new feature backported to maintained releases? If this is a new feature, it needs an entry in Whant's New and a versionchanged directive in the module documentation. If this is a bug fix, it likely doesn't need any special documenting.