skbuff: render the checksum comment to documentation

Long time ago Tom added a giant comment to skbuff.h explaining
checksums. Now that we have a place in Documentation for skbuff
docs we should render it. Sprinkle some markup while at it.

Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>