Missing items in the documention for the mem module

[BusyBeaver-42]

There’s a small issue in the documentation for mem::can_dereference and similar functions. It refers to items in kani's mem module using crate::mem, but these items aren’t actually present in the documentation for the mem module of kani. Instead, they’re found in the documentation for the mem module of kani_core, where functions like can_dereference are defined.