srcu: Document __srcu_read_{,un}lock_fast() implicit RCU readers

srcu: Document __srcu_read_{,un}lock_fast() implicit RCU readers

This commit documents the implicit RCU readers that are implied by the
this_cpu_inc() and atomic_long_inc() operations in __srcu_read_lock_fast()
and __srcu_read_unlock_fast().  While in the area, fix the documentation
of the memory pairing of atomic_long_inc() in __srcu_read_lock_fast().

[ paulmck: Apply Joel Fernandes feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <bpf@vger.kernel.org>