tools/memory-model: Update some warning labels

Some of the warning labels used in the LKMM are unfortunately
ambiguous.  In particular, the same warning is used for both an
unmatched rcu_read_lock() call and for an unmatched rcu_read_unlock()
call.  Likewise for the srcu_* equivalents.  Also, the warning about
passing a wrong value to srcu_read_unlock() -- i.e., a value different
from the one returned by the matching srcu_read_lock() -- talks about
bad nesting rather than non-matching values.

Let's update the warning labels to make their meanings more clear.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>