Fix broken link and stale descriptions in doc (#1139)

docs/userguide/src/portingguide/howto/nogc.md

@@ -64,8 +64,8 @@ The `VMBinding` trait is a "meta-trait" (i.e. a trait that encapsulates other tr
   3. [`ObjectModel`](https://docs.mmtk.io/api/mmtk/vm/trait.ObjectModel.html): This trait implements the runtime's object model. The object model includes object metadata such as mark-bits, forwarding-bits, etc.; constants regarding assumptions about object addresses; and functions to implement copying objects, querying object sizes, etc. You should ***carefully*** implement and understand this as it is a key trait on which many things depend. We will go into more detail about this trait in the [object model section](#object-model).
   4. [`ReferenceGlue`](https://docs.mmtk.io/api/mmtk/vm/trait.ReferenceGlue.html): This trait implements runtime-specific finalization and weak reference processing methods. Note that each runtime has its own way of dealing with finalization and reference processing, so this is often one of the trickiest traits to implement.
   5. [`Scanning`](https://docs.mmtk.io/api/mmtk/vm/trait.Scanning.html): This trait implements object scanning functions such as scanning mutator threads for root pointers, scanning a particular object for reference fields, etc.
-  6. [`Slot`](https://docs.mmtk.io/api/mmtk/vm/slot/trait.Slot.html): This trait implements what an edge in the object graph looks like in the runtime. This is useful as it can abstract over compressed or tagged pointers. If an edge in your runtime is indistinguishable from an arbitrary address, you may set it to the [`Address`](https://docs.mmtk.io/api/mmtk/util/address/struct.Address.html) type.
-  7. [`MemorySlice`](https://docs.mmtk.io/api/mmtk/vm/edge_shape/trait.MemorySlice.html): This trait implements functions related to memory slices such as arrays. This is mainly used by generational collectors.
+  6. [`Slot`](https://docs.mmtk.io/api/mmtk/vm/slot/trait.Slot.html): This trait implements a slot in an object, on the stack or other places (such as global variables). If a slot in your runtime simply holds the address of an object (or 0 for NULL references), you may use the [`SimpleSlot`](https://docs.mmtk.io/api/mmtk/vm/slot/struct.SimpleSlot.html) type. But if your VM uses tagged pointers or compressed pointers, you will need to implement it manually.
+  7. [`MemorySlice`](https://docs.mmtk.io/api/mmtk/vm/slot/trait.MemorySlice.html): This trait implements functions related to memory slices such as arrays. This is mainly used by generational collectors.
 
 For the time-being we can implement all the above traits via `unimplemented!()` stubs. If you are using the Dummy VM binding as a starting point, you will have to edit some of the concrete implementations to `unimplemented!()`. Note that you should change the type that implements `VMBinding` from `DummyVM` to an appropriately named type for your runtime. For example, the OpenJDK binding defines the zero-struct [`OpenJDK`](https://github.com/mmtk/mmtk-openjdk/blob/54a249e877e1cbea147a71aafaafb8583f33843d/mmtk/src/lib.rs#L139-L162) which implements the `VMBinding` trait.