rcu: Add lockdep checks and kernel-doc header to rcu_softirq_qs()

rcu: Add lockdep checks and kernel-doc header to rcu_softirq_qs()

There is some indications that rcu_softirq_qs() might be more generally
used than anticipated.  This commit therefore adds some lockdep assertions
and some cautionary tales in a new kernel-doc header.

Link: https://lore.kernel.org/all/Zd4DXTyCf17lcTfq@debian.debian/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Yan Zhai <yan@cloudflare.com>
Cc: <netdev@vger.kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>