org.texi (Installation): Fix documentation bug

* org.texi (Installation): Be more clear on why installing Org
through ELPA should be done without loading any Org file.

This is a very common pitfall that needs to be considered a
documentation bug.

doc/org.texi

@@ -878,7 +878,11 @@ 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}.
-You need to do this in a session where no @code{.org} file has been visited.
+
+@noindent @b{Important}: you need to do this in a session where no @code{.org} file has
+been visited, i.e. where no Org built-in function have been loaded.
+Otherwise autoload Org functions will mess up the installation.
+
 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,