doc: Emphasize that "toy" RCU requires recursive rwlock

familiar locking primitives.  Its overhead makes it a non-starter for

real-life use, as does its lack of scalability.  It is also unsuitable

for realtime use, since it allows scheduling latency to "bleed" from

one read-side critical section to another.

one read-side critical section to another.  It also assumes recursive

reader-writer locks:  If you try this with non-recursive locks, and

you allow nested rcu_read_lock() calls, you can deadlock.

However, it is probably the easiest implementation to relate to, so is

a good starting point.