Clarify doc string of 'file-name-sans-extension'

* lisp/files.el (file-name-sans-extension, file-name-extension):
Clarify in the doc strings what is the extension, and which
leading period doesn't count.  (Bug#23643)

lisp/files.el

@@ -4257,8 +4257,8 @@ the group would be preserved too."
 
 (defun file-name-sans-extension (filename)
   "Return FILENAME sans final \"extension\".
-The extension, in a file name, is the part that follows the last `.',
-except that a leading `.', if any, doesn't count."
+The extension, in a file name, is the part that begins with the last `.',
+except that a leading `.' of the file name, if there is one, doesn't count."
   (save-match-data
     (let ((file (file-name-sans-versions (file-name-nondirectory filename)))
 	  directory)
@@ -4273,15 +4273,16 @@ except that a leading `.', if any, doesn't count."
 
 (defun file-name-extension (filename &optional period)
   "Return FILENAME's final \"extension\".
-The extension, in a file name, is the part that follows the last `.',
-excluding version numbers and backup suffixes,
-except that a leading `.', if any, doesn't count.
+The extension, in a file name, is the part that begins with the last `.',
+excluding version numbers and backup suffixes, except that a leading `.'
+of the file name, if there is one, doesn't count.
 Return nil for extensionless file names such as `foo'.
 Return the empty string for file names such as `foo.'.
 
-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 \"\"."
   (save-match-data
     (let ((file (file-name-sans-versions (file-name-nondirectory filename))))
       (if (and (string-match "\\.[^.]*\\'" file)