doc: networking: prepare offload documents for conversion into RST

Add small number of markups which are sufficient for conversion
into reStructuredText.

Unfortunately there was necessary to restructure all sections
in checksum-offloads.txt file and create paragraphs separated
by newline. There also must not be a space at the
beginning of paragpraph.

There are no semantic changes.

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