skbuff.h: Improve the checksum related comments

Fixed the punctuation and some typos.
Improved some sentences with minor changes.

No change of semantics or code.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>