org.texi (Installation): Detail instructions when installing through ELPA

* org.texi (Installation): When installing Org from ELPA, users
should do this from an Emacs session where no .org file has been
visited.

doc/org.texi

@@ -855,10 +855,11 @@ We @b{strongly recommend} to stick to a single installation method.
 
 Recent Emacs distributions include a packaging system which lets you install
 Elisp libraries.  You can install Org with @kbd{M-x package-install RET org}.
-To make sure your Org configuration is well taken into account, initialize
-the package system with @code{(package-initialize)} before setting any Org
-option.  If you want to use Org's package repository, check out the
-@uref{http://orgmode.org/elpa.html, Org ELPA page}.
+You need to do this in a session where no @code{.org} file has been visited.
+Then, to make sure your Org configuration is taken into account, initialize
+the package system with @code{(package-initialize)} in your @file{.emacs}
+before setting any Org option.  If you want to use Org's package repository,
+check out the @uref{http://orgmode.org/elpa.html, Org ELPA page}.
 
 @subsubheading Downloading Org as an archive