[Chapel Merge] Update launcher docs to reflect current defaults

Branch: refs/heads/master
Revision: 0904561
Author: ronawho
Log Message:

Merge pull request #17376 from ronawho/update-launcher-docs

Update launcher docs to reflect current defaults

[reviewed by @gbtitus]

Update docs to reflect amudprun priority and that a slurm launcher will
be used on cray-cs systems if srun/salloc are available.

Modified Files:
M doc/rst/usingchapel/launcher.rst

Compare: https://github.com/chapel-lang/chapel/compare/7d01bba85e53...09045617ac6b