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>

Documentation/networking/skbuff.rst

@@ -29,3 +29,9 @@ dataref and headerless skbs
 
 .. kernel-doc:: include/linux/skbuff.h
    :doc: dataref and headerless skbs
+
+Checksum information
+--------------------
+
+.. kernel-doc:: include/linux/skbuff.h
+   :doc: skb checksums