torture: Update initrd documentation

The rcu-test-image.txt documentation covers a very uncommon case where
a real userspace environment is required.  However, someone reading this
document might reasonably conclude that this is in fact a prerequisite.
In addition, the initrd.txt file mentions dracut, which is no longer used.
This commit therefore provides the needed updates.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>