Document insert-image-file's return value (Bug#32978)

* lisp/image-file.el (insert-image-file): Document return value.

lisp/image-file.el

@@ -97,8 +97,9 @@ the variable is set using \\[customize]."
 ;;;###autoload
 (defun insert-image-file (file &optional visit beg end replace)
   "Insert the image file FILE into the current buffer.
-Optional arguments VISIT, BEG, END, and REPLACE are interpreted as for
-the command `insert-file-contents'."
+Optional arguments VISIT, BEG, END, and REPLACE are interpreted
+as for the command `insert-file-contents'.  Return list of
+absolute file name and number of characters inserted."
   (let ((rval
 	 (image-file-call-underlying #'insert-file-contents-literally
 				     'insert-file-contents