Document pointer cast semantics

[matthewjasper]

#13 documents which casts are possible, as well as the semantics of the casts that don't involve pointers. For pointers it's not clear if a complete set of semantics can be given until the unsafe guidelines/memory model are determined, but maybe some statements can be made.

[bluss]

Including *const T as *const u8 casts where T: ?Sized.