More doc fixes in package.el (Bug#37544)

More doc fixes in package.el (Bug#37544)

* lisp/emacs-lisp/package.el (package-menu-hide-low-priority)
(package-pinned-packages, package-load-descriptor)
package-archive-version, package-archive-contents)
package--read-archive-file, package-read-archive-contents)
(package-unsigned-archives, package-read-all-archive-contents)
(package--download-and-read-archives, package-install):
* lisp/subr.el (package--description-file):
* test/lisp/emacs-lisp/package-tests.el: Doc fixes.