docs: networking: Add title caret and missing doc

Resolving a couple of Sphinx documentation warnings
that are generated in the networking section.

- WARNING: document isn't included in any toctree
- WARNING: Title underline too short.

Signed-off-by: Adam Zerella <adam.zerella@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>