Doc fixes for package.el.

Doc fixes for package.el.

* emacs-lisp/package.el (package--builtins, package-alist)
(package-load-descriptor, package-built-in-p, package-activate)
(define-package, package-installed-p)
(package-compute-transaction, package-buffer-info)
(package--push): Doc fix.  Distinguish more clearly between
version strings and version lists.