mptcp: fix kdoc warnings

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 <geliang@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260205-net-mptcp-misc-fixes-6-19-rc8-v2-3-c2720ce75c34@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>