Clarify role of Node.NodeBuilder.

Prompted in part by
https://github.com/ipld/go-ipld-prime/pull/21#issuecomment-518115245 .

Turns out some of the stuff our earliest Node implementations do are
actually pretty unusual.  Many other features have more strictures.
Therefore, it's important to specify the behavior such that the
features that have to operate within more restrictions can still
comply, and it's clear that anything else shouldn't be expected!

(Perhaps we should just... make the ipldfree.Node implementations not
do the flexible things they do right now.  That package is starting
to look in need of some renovation anyway.  We'll see.)
Signed-off-by: Eric Myhre <hash@exultant.us>

node.go

@@ -79,6 +79,30 @@ type Node interface {
 	// e.g. traversal.Transform, for doing tree updates while keeping the
 	// existing implementation preferences and doing as many operations
 	// in copy-on-write fashions as possible.)
+	//
+	// ---
+	//
+	// More specifically, the contract of a NodeBuilder returned by this method
+	// is that it should be able to "replace" this node with a new one of
+	// similar properties.
+	// E.g., for a string, the builder must be able to build a new string.
+	// For a map, the builder must be able to build a new map.
+	// For a *struct* (when using typed nodes), the builder must be able to
+	// build new structs of the name type.
+	// Note that the promise doesn't extend further: there's no requirement
+	// that the builder be able to build maps if this node's kind is "string"
+	// (you can see why this lack-of-contract is important when considering
+	// typed nodes: if this node has a struct type, then should the builder
+	// be able to build other structs of different types?  Of course not;
+	// there'd be no way to define which other types to build!).
+	// For nulls, this means the builder doesn't have to do much at all!
+	//
+	// (Some Nodes may return a NodeBuilder that can be used for much more
+	// than replacing their own kind: for example, Node implementations from
+	// the ipldfree package tend to return a NodeBuilder than can build any
+	// 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.)
 	NodeBuilder() NodeBuilder
 }