doc: networking: shorten the main title in offloads documents

The titles do not look very nice in the table of contents generated by
Sphinx.

I also think it is obvious that the documents are describing offloads
in the Linux Networking Stack.

Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>