DEV_DOCS: Document the shared Cram setup scripts

[victorlin]

Description of proposed changes

Add a note on this now-consistent practice.

Related issue(s)

• Use shared setup script for all Cram tests #1637
• Set pipefail for Cram tests #1636 (review)

Checklist

• Automated checks pass Changes not checked
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs