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>

Documentation/networking/index.rst

@@ -97,6 +97,7 @@ Contents:
    sctp
    secid
    seg6-sysctl
+   skbuff
    smc-sysctl
    statistics
    strparser

Documentation/networking/skbuff.rst

@@ -0,0 +1,37 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+struct sk_buff
+==============
+
+:c:type:`sk_buff` is the main networking structure representing
+a packet.
+
+Basic sk_buff geometry
+----------------------
+
+.. kernel-doc:: include/linux/skbuff.h
+   :doc: Basic sk_buff geometry
+
+Shared skbs and skb clones
+--------------------------
+
+:c:member:`sk_buff.users` is a simple refcount allowing multiple entities
+to keep a struct sk_buff alive. skbs with a ``sk_buff.users != 1`` are referred
+to as shared skbs (see skb_shared()).
+
+skb_clone() allows for fast duplication of skbs. None of the data buffers
+get copied, but caller gets a new metadata struct (struct sk_buff).
+&skb_shared_info.refcount indicates the number of skbs pointing at the same
+packet data (i.e. clones).
+
+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