various: Update/fix some textual material and code comments.

Of these, only struct net_ipv6_nbr_data::send_ns is a descriptive
change:

send_ns is used for timing Neighbor Solicitations in general, not
just for DAD.

The rest are typo/grammar fixes.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
10 files changed