[Chapel Merge] Add docs for CHPL_RT_UNWIND

Branch: refs/heads/main
Revision: 75dd441
Author: aconsroe-hpe
Link: Unavailable
Log Message:

Merge pull request #18590 from aconsroe-hpe/chpl-rt-unwind-docs

Add docs for CHPL_RT_UNWIND

Reviewed by @bmcdonald3

The environment variable CHPL_RT_UNWIND was previously undocumented. This adds an explanation in the section "Using Chapel" > "Executing Chapel Programs"

Relates to #5320

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

Compare: https://github.com/chapel-lang/chapel/compare/df5b8cb46fac...75dd441d360b