mptcp: fix kdoc warnings

The following warnings were visible:

$ ./scripts/kernel-doc -Wall -none \
net/mptcp/ include/net/mptcp.h include/uapi/linux/mptcp*.h \
include/trace/events/mptcp.h
Warning: net/mptcp/token.c:108 No description found for return value of 'mptcp_token_new_request'
Warning: net/mptcp/token.c:151 No description found for return value of 'mptcp_token_new_connect'
Warning: net/mptcp/token.c:246 No description found for return value of 'mptcp_token_get_sock'
Warning: net/mptcp/token.c:298 No description found for return value of 'mptcp_token_iter_next'
Warning: net/mptcp/protocol.c:4431 No description found for return value of 'mptcp_splice_read'
Warning: include/uapi/linux/mptcp_pm.h:13 missing initial short description on line:
* enum mptcp_event_type

Address all of them: either by using the 'Return:' keyword, or by adding
a missing initial short description.

The MPTCP CI will soon report issues with kdoc to avoid introducing new
issues and being flagged by the Netdev CI.

Reviewed-by: Geliang Tang <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/netlink/specs/mptcp_pm.yaml

···
     type: enum
     name: event-type
     enum-name: mptcp-event-type
+    doc: Netlink MPTCP event types
     name-prefix: mptcp-event-
     entries:
       -

include/uapi/linux/mptcp_pm.h

···
 #define MPTCP_PM_VER	1
 
 /**
- * enum mptcp_event_type
+ * enum mptcp_event_type - Netlink MPTCP event types
  * @MPTCP_EVENT_UNSPEC: unused event
  * @MPTCP_EVENT_CREATED: A new MPTCP connection has been created. It is the
  *   good time to allocate memory and send ADD_ADDR if needed. Depending on the

net/mptcp/token.c

···
  * It creates a unique token to identify the new mptcp connection,
  * a secret local key and the initial data sequence number (idsn).
  *
- * Returns 0 on success.
+ * Return: 0 on success.
  */
 int mptcp_token_new_request(struct request_sock *req)
 {
···
  * the computed token at a later time, this is needed to process
  * join requests.
  *
- * returns 0 on success.
+ * Return: 0 on success.
  */
 int mptcp_token_new_connect(struct sock *ssk)
 {
···
  * This function returns the mptcp connection structure with the given token.
  * A reference count on the mptcp socket returned is taken.
  *
- * returns NULL if no connection with the given token value exists.
+ * Return: NULL if no connection with the given token value exists.
  */
 struct mptcp_sock *mptcp_token_get_sock(struct net *net, u32 token)
 {
···
  * @s_slot: start slot number
  * @s_num: start number inside the given lock
  *
- * This function returns the first mptcp connection structure found inside the
- * token container starting from the specified position, or NULL.
+ * Description:
+ *   On successful iteration, the iterator is moved to the next position and a
+ *   reference to the returned socket is acquired.
  *
- * On successful iteration, the iterator is moved to the next position and
- * a reference to the returned socket is acquired.
+ * Return:
+ *   The first mptcp connection structure found inside the token container
+ *   starting from the specified position, or NULL.
  */
 struct mptcp_sock *mptcp_token_iter_next(const struct net *net, long *s_slot,
 					 long *s_num)