Improve doc strings of a recent commit

Improve doc strings of a recent commit

* lisp/emacs-lisp/generator.el (iter-defun):
* lisp/emacs-lisp/comp.el (comp-clean-up-stale-eln):
* lisp/emacs-lisp/bytecomp.el (byte-compile-lapcode):
* lisp/emacs-lisp/autoload.el (autoload-insert-section-header):
Include description of arguments in the doc string's first line.