SLURM doc updates
Merge pull request #586 from vasslitvinov/adjust-readmes-for-SLURM-launcher

SLURM doc updates - same as #585 on master.

* Added the requirement to set CHPL_LAUNCHER_WALLTIME for slurm-gasnetrun_ibv launcher.

While there:

* Noted that CHPL_LAUNCHER gets set to none when slurm and aprun are both in user's path.

* Streamlined slurm notes.

* Tweaked the formatting of how CHPL_LAUNCHER is chosen by default.

Reviewed by Elliot.