[Chapel Merge] Update HPE Cray EX platform to recommend loading C

Branch: refs/heads/main
Revision: 0f6afce
Author: gbtitus
Log Message:

Merge pull request #18416 from gbtitus/cray-ex-pmi

Update HPE Cray EX platform to recommend loading Cray PMI modules.

(Reviewed by @ronawho.)

Add a recommendation to load the Cray PMI modules before building Chapel
programs on HPE Cray EX systems, to forestall problems due to a lack of
KVS resources in the default PMI.

While here, move the HPE Cray EX section of Special Notes before the
Cray XC section of same.

Modified Files:
M doc/rst/platforms/cray.rst

Compare: https://github.com/chapel-lang/chapel/compare/a310da17173d...0f6afcef8394