[PR #14467/7df5d80f backport][9.0.x] docs: clarify subtests progress output

[patchback]

This is a backport of PR #14467 as merged into main (7df5d80).

Summary

• Render the subtests example output as plain text so the docs do not color every u progress marker as passing.
• Add a short note explaining that the compact u marker is used for both passed and failed subtests, and that the summary shows which subtests failed.
• Add a short changelog fragment for the documentation clarification.

Closes #13902.

Testing

• git diff --check
• uv run --python 3.14 --with tox tox -e docs -- -q

Notes

• The commit includes an AI co-author trailer, per the project PR template.