Documentation: preempt-locking: Use better example

Documentation: preempt-locking: Use better example

The existing wording implies that the use of spin_unlock whilst irqs are
disabled might trigger a reschedule. However the preemptible() test in
preempt_schedule will prevent a reschedule if irqs are disabled.

Lets improve the clarity of this wording to change the example from
spin_unlock to cond_resched() and cond_resched_lock() as these are
functions that will trigger a reschedule if the preempt count is 0 without
testing that irqs are disabled.

Also remove the 'Last Updated' line as this is not up to date and better
tracked via GIT.

Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>