doc/net/l2tpv3: Update boolean fields' description to avoid short-form use

doc/net/l2tpv3: Update boolean fields' description to avoid short-form use

The short-form boolean options has been deprecated since v6.0 (refer
to docs/about/deprecated.rst).

Update the description and example of boolean fields in l2tpv3 option to
avoid deprecation warning.

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>