Clarify what type of objects can be deleted

[carpawell]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I see some comments in implementation code cannot be proved by API references. E.g. we do not allow tombstone deletion, lock object deletion but I cannot find any words about it near Tombstone or Delete.

Describe the solution you'd like

Clarify that what exact is not possible to delete.

Describe alternatives you've considered

Keep as is, it allows some free space in implementations but also makes a dev sometimes do not understand how implement deletion code.

Additional context

https://github.com/nspcc-dev/neofs-node/blob/c4bdae16ab06a49349cc446f897dc00dc6420831/pkg/local_object_storage/metabase/inhume.go#L131-L132