documentation/networking: update l2tp docs

l2tp no longer uses sk_user_data in tunnel sockets and now manages
tunnel/session lifetimes slightly differently. Update docs to cover
this.

CC: linux-doc@vger.kernel.org
CC: corbet@lwn.net
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>