doc: networking: qemu_setup: Update details and add DNS information

Majority of "accessing Internet" setup is the same for both QEMU and
real boards connected to host, mention that explicitly. Be more
precise that networking settings require net_app_settings API, not
entire net_app. Finally, add information about DNS setup for the
samples requiring it.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>