Clarify page-only delivery and artifact verification

delivery.go

@@ -90,9 +90,12 @@ type DeliveryAsset struct {
 
 // DeliveryRequest identifies a local tagged checkout and its explicit
 // destination. Build selects a managed build; Prebuilt supplies caller-owned
-// build output instead. They cannot both be set. Either may be nil for
-// page-only delivery when both are nil. Build with no targets, or an empty
-// Prebuilt result, also performs page-only delivery.
+// build output instead. They cannot both be set. The ordinary page-only request
+// leaves both nil. A Build with no targets or a matching Prebuilt result with
+// no targets or artifacts also performs page-only delivery. Any supplied
+// Prebuilt must match the repository, module directory and path, tag, commit,
+// and version; a zero BuildResult does not supply that identity. Source and
+// artifact requirements for binaries apply when binary work is selected.
 type DeliveryRequest struct {
 	RepositoryDir string // directory within the tagged source Git checkout.
 	// ModuleDir selects a repository-relative module directory; empty selects