Add description of Node.NodeBuilder <-> ADL relationship.

Mind, I don't know if all the details of this have been worked out.
In particular, where to get the LinkStorer and other fun stuff passed
through this interface might be a fun dance (if, indeed, such a thing
ends up needed -- this hasn't been written yet!).  We'll see.
One thing is clear: we want this contract here for all the same
already-known reasons: a traversal.Transform over a structure using
an ADL should "just work".
Signed-off-by: Eric Myhre <hash@exultant.us>

node.go

@@ -103,6 +103,12 @@ type Node interface {
 	// other ipldfree.Node (e.g. even the builder obtained from a string node
 	// will be able to build maps).  This is not required by the contract;
 	// such packages only do so out of internal implementation convenience.)
+	//
+	// This "able to replace" behavior also has a specific application regarding
+	// nodes implementing Advanced Data Layouts: it means that the NodeBuilder
+	// returned by this method must produce a new Node using that same ADL.
+	// For example, if a Node is a map implemented by some sort of HAMT, its
+	// NodeBuilder must also produce a new HAMT.
 	NodeBuilder() NodeBuilder
 }