Remove upload functionality of package-x from the elisp manual

Remove upload functionality of package-x from the elisp manual

Suggested by Stefan Monnier.
Ref: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19537#8

* doc/lispref/package.texi (Package Archives): Don't document
package-x upload functions in the elisp manual, since they are not
very commonly used.  (Bug#19537)
* lisp/emacs-lisp/package-x.el (package-archive-upload-base)
(package-upload-buffer, package-upload-file): Add to the doc strings
any details removed from the elisp manual that would otherwise be
missing.