diff --git a/vendor/cmd/go/internal/module/module.go b/vendor/cmd/go/internal/module/module.go
index 1e8f74c..db40645 100644
--- a/vendor/cmd/go/internal/module/module.go
+++ b/vendor/cmd/go/internal/module/module.go
@@ -17,7 +17,14 @@
 
 // A Version is defined by a module path and version pair.
 type Version struct {
-	Path    string
+	Path string
+
+	// Version is usually a semantic version in canonical form.
+	// There are two exceptions to this general rule.
+	// First, the top-level target of a build has no specific version
+	// and uses Version = "".
+	// Second, during MVS calculations the version "none" is used
+	// to represent the decision to take no version of a given module.
 	Version string
 }
 
diff --git a/vendor/cmd/go/internal/mvs/mvs.go b/vendor/cmd/go/internal/mvs/mvs.go
index 9e7816d..dccea00 100644
--- a/vendor/cmd/go/internal/mvs/mvs.go
+++ b/vendor/cmd/go/internal/mvs/mvs.go
@@ -281,6 +281,10 @@
 
 // Downgrade returns a build list for the target module
 // in which the given additional modules are downgraded.
+//
+// The versions to be downgraded may be unreachable from reqs.Latest and
+// reqs.Previous, but the methods of reqs must otherwise handle such versions
+// correctly.
 func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([]module.Version, error) {
 	list, err := reqs.Required(target)
 	if err != nil {
diff --git a/vendor/cmd/go/internal/vgo/load.go b/vendor/cmd/go/internal/vgo/load.go
index 67f878a..4ef52a1 100644
--- a/vendor/cmd/go/internal/vgo/load.go
+++ b/vendor/cmd/go/internal/vgo/load.go
@@ -397,6 +397,8 @@
 	modFile.AddRequire(root, info.Version)
 }
 
+// mvsReqs implements mvs.Reqs for vgo's semantic versions, with any exclusions
+// or replacements applied internally.
 type mvsReqs struct {
 	extra []module.Version
 	cache par.Cache
@@ -532,6 +534,8 @@
 	return v1
 }
 
+// Latest returns the latest tagged version of the module at path,
+// or the latest untagged version if no version is tagged.
 func (*mvsReqs) Latest(path string) (module.Version, error) {
 	// Note that query "latest" is not the same as
 	// using repo.Latest.
@@ -557,6 +561,8 @@
 	return repo.Versions("")
 }
 
+// Previous returns the tagged version of m.Path immediately prior to
+// m.Version, or version "none" if no prior version is tagged.
 func (*mvsReqs) Previous(m module.Version) (module.Version, error) {
 	list, err := versions(m.Path)
 	if err != nil {