flatpak.org/setup/: Modify the Restart section to indicate that packages can be run GNU/Linux distros in live mode

Checklist

[X] I agree to follow the Code of Conduct that this project adheres to.
[X] I have searched the issue tracker for a bug that matches the one I want to file, without success.
[X] If this is an issue with a particular app, I have tried filing it in the appropriate issue tracker for the app (e.g. under https://github.com/flathub/) and determined that it is an issue with Flatpak itself.
[X] This issue is not a report of a security vulnerability (see here if you need to report a security issue).

Flatpak version

1.12.7-1

What Linux distribution are you using?

Ubuntu

Linux distribution version

22.04

What architecture are you using?

x86_64

How to reproduce

Ubuntu [MATE] 23.04 and earlier

Follow https://flatpak.org/setup/Ubuntu EXCEPT the Restart section

$ sudo apt install flatpak
$ flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo

Note that the directories 

'/var/lib/flatpak/exports/share'
'/home/x/.local/share/flatpak/exports/share'

are not in the search path set by the XDG_DATA_DIRS environment variable, so
applications installed by Flatpak may not appear on your desktop until the
session is restarted.
$ flatpak install flathub org.libretro.RetroArch
$ flatpak run org.libretro.RetroArch

RetroArch opens

Ubuntu [MATE] 24.04 (issue)

Follow https://flatpak.org/setup/Ubuntu EXCEPT the Restart section

$ sudo apt install flatpak
$ flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo

Note that the directories 

'/var/lib/flatpak/exports/share'
'/home/x/.local/share/flatpak/exports/share'

are not in the search path set by the XDG_DATA_DIRS environment variable, so
applications installed by Flatpak may not appear on your desktop until the
session is restarted.
$ flatpak install flathub org.libretro.RetroArch
$ flatpak run org.libretro.RetroArch

Note that the directories 

'/var/lib/flatpak/exports/share'
'/home/ubuntu-mate/.local/share/flatpak/exports/share'

are not in the search path set by the XDG_DATA_DIRS environment variable, so
applications installed by Flatpak may not appear on your desktop until the
session is restarted.

bwrap: Creating new namespace failed: Permission denied
error: ldconfig failed, exit status 256

Expected Behavior

As @matthiasclasen pointed out, flatpak emits a warning.

I expected pages on https://flatpak.org/setup/ that says (probably from a template):

"To complete setup, restart your system." to mention the output from "flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo" (see above) instead:
"Applications installed by Flatpak may not appear on your desktop until the session is restarted. However, Ubuntu 24.04 requires restart."

This will help people find out that Flatpak can be run GNU/Linux distros in live mode.

Actual Behavior

Most pages in https://flatpak.org/setup/ says "To complete setup, restart your system.":

Additional Information

"Random idea: We can make flatpak check if the environment looks ok, and if it doesn't, emit a warning." @matthi

https://github.com/flatpak/flatpak/issues/1256#issuecomment-452163683

flatpak / flatpak.github.io

flatpak.org/setup/: Modify the Restart section to indicate that packages can be run GNU/Linux distros in live mode #619