documentation: Document call_rcu() safety mechanisms and limitations

The call_rcu() family of primitives will take action to accelerate
grace periods when the number of callbacks pending on a given CPU
becomes excessive.  Although this safety mechanism can be useful,
it is no substitute for users of call_rcu() having rate-limit controls
in place.  This commit adds this nuance to the documentation.

Reported-by: "Michael S. Tsirkin" <[email protected]>
Reported-by: Gleb Natapov <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>

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