Print warning that task/cray must be listed before task/cgroup in TaskPlugin

task/cray's _get_numa_nodes() function needs to run before task/cgroup
cleans up the cgroup hierarchies, otherwise ALPS memory compaction will
never run.

Also move task_p_add_pid() outside the #ifdef HAVE_NATIVE_CRAY
block so that the plugin will load (albeit without any functionality)
on non-Cray systems for testing purposes.

Revise documentation and provided slurm.conf templates as well.

Bug 3154.