Merge branch 'docs-document-some-aspects-of-struct-sk_buff'

Jakub Kicinski says:

====================
docs: document some aspects of struct sk_buff

This small set creates a place to render sk_buff documentation,
documents one random thing (data-only skbs) and converts the big
checksum comment to kdoc.
====================

Link: https://lore.kernel.org/r/20220323233715.2104106-1-kuba@kernel.org/
Link: https://lore.kernel.org/r/20220324231312.2241166-1-kuba@kernel.org/
Link: https://lore.kernel.org/r/20220509160456.1058940-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>