address get manifest issue with oci. namespace; and comment descriptions

Signed-off-by: Mike Brown <brownwm@us.ibm.com>

manifest/schema2/manifest.go

@@ -79,7 +79,7 @@ func (m Manifest) References() []distribution.Descriptor {
 	return references
 }
 
-// Target returns the target of this signed manifest.
+// Target returns the target of this manifest.
 func (m Manifest) Target() distribution.Descriptor {
 	return m.Config
 }