doc: application: tweak experimental features text

We use a specific experimental Kconfig option when describing the
behavior of CONFIG_WARN_EXPERIMENTAL.

However, this option may not be experimental forever. This therefore
may go stale. Rather than try to keep generic documentation about
experimental options up to date with whatever the bluetooth subsystem
happens to consider experimental or not, use a placeholder CONFIG_FOO
instead of a real option.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>