docs: networking: add tracepoint info to l2tp.rst

Update l2tp.rst to:

 * remove information about the obsolete debug flags and their use
 * include information about tracepoints for l2tp

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
1 file changed