sched_ext: Clarify ops.select_cpu() for single-CPU tasks

Update ops.select_cpu() documentation to clarify that this method is not
called for tasks that are restricted to run on a single CPU, as these
tasks do not have the option to select a different CPU.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>