PPPoL2TP: Add more code snippets

The existing documentation was not telling that one has to create a PPP
channel and a PPP interface to get PPPoL2TP data offloading working.

Also, tunnel switching was not mentioned, so that people were thinking
it was not supported, while it actually is.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Tom Parkin <tparkin@katalix.com>
Link: https://lore.kernel.org/r/20240217211425.qj576u3jmaa6yidf@begin
Signed-off-by: Jakub Kicinski <kuba@kernel.org>