update to use typed sensitive marks

internal/lang/marks/marks.go

@@ -4,10 +4,15 @@ import (
 	"strings"
 )
 
+// valueMarks allow creating strictly typed values for use as cty.Value marks.
+// The variable name for new values should be the title-cased format of the
+// value to better match the GoString output for debugging.
 type valueMark string
 
 func (m valueMark) GoString() string {
 	return "marks." + strings.Title(string(m))
 }
 
+// Sensitive indicates that this value is marked as sensitive in the context of
+// Terraform.
 var Sensitive = valueMark("sensitive")