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>

Documentation/networking/device_drivers/index.rst

@@ -23,6 +23,7 @@ Contents:
    intel/ice
    google/gve
    mellanox/mlx5
+   netronome/nfp
    pensando/ionic
 
 .. only::  subproject and html

Documentation/networking/j1939.rst

@@ -272,7 +272,7 @@ supported flags are:
 * MSG_DONTWAIT, i.e. non-blocking operation.
 
 recvmsg(2)
-^^^^^^^^^
+^^^^^^^^^^
 
 In most cases recvmsg(2) is needed if you want to extract more information than
 recvfrom(2) can provide. For example package priority and timestamp. The