net: add missing documentation in linux/skbuff.h

This patch adds missing documentation for some inline functions on
linux/skbuff.h. The patch is incomplete and a lot more can be added,
just wondering if it's of interest of the netdev developers.

Also fixed some whitespaces.

Signed-off-by: Pedro Tammela <pctammela@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>