Mention that by default there is no server side decoration for Qt apps

Current documentation:

The GNOME X11 session has been removed from Fedora Linux. Users previously using GNOME on X11 will be transparently upgraded to the GNOME Wayland session. X11 applications are still supported on GNOME.

For the benefit of previous GNOME X11 users and users of other desktop environments wanting to try GNOME, please

• mention that by default, Qt apps will not have any windows buttons Screenshot.
• provide fixes/workarounds, e.g. QT_QPA_PLATFORM=xcb, or
• mention upfront that GNOME is not intended to support Qt apps