* lisp/emacs-lisp/package.el (package-initialize): Set enable-at-startup

* lisp/emacs-lisp/package.el (package-initialize): Set enable-at-startup

When `package-initialize' is called as part of loading the init file,
the user probably doesn't want it to be called again afterwards. In this
situation, `package-initialize' now sets `package-enable-at-startup' to
nil to prevent that. The user can have the old behaviour by setting this
variable to t after the call to `package-initialize'.  (Bug#21423)

* doc/emacs/package.texi (Package Installation): Document it

* doc/lispref/package.texi (Packaging Basics): Document it

* etc/NEWS: Document it