Document pem.ModTime().

pkg/pem/pem.go

@@ -158,6 +158,9 @@ func (pemfiles *Files) Path(pem *File) string {
 	return filepath.Join(pemfiles.Base, pem.Basename())
 }
 
+// ModTime returns the modification time of PEM files metadata -- the
+// time of the most recent Write() or Delete() call that resulted in a
+// mutation of the PEM files, or initalization of the metadata.
 func (pemfiles *Files) ModTime() time.Time {
 	pemfiles.mtx.RLock()
 	defer pemfiles.mtx.RUnlock()