Documentation/networking: update l2tp docs

Kernel documentation of L2TP has not been kept up to date and lacks
coverage of some L2TP APIs. While addressing this, refactor to improve
readability, separating the parts which focus on user APIs and
internal implementation into sections.

Changes in v2:

 - fix checkpatch warnings about trailing whitespace and long lines
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>