-If PERIOD is non-nil, then the returned value includes the period
-that delimits the extension, and if FILENAME has no extension,
-the value is \"\"."
+By default, the returned value excludes the period that starts the
+extension, but if the optional argument PERIOD is non-nil, the period
+is included in the value, and in that case, if FILENAME has no
+extension, the value is \"\"."