rcu: Comment why callbacks migration can't wait for CPUHP_RCUTREE_PREP

The callbacks migration is performed through an explicit call from
the hotplug control CPU right after the death of the target CPU and
before proceeding with the CPUHP_ teardown functions.

This is unusual but necessary and yet uncommented. Summarize the reason
as explained in the changelog of:

	a58163d8 (rcu: Migrate callbacks earlier in the CPU-offline timeline)

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>