Docs: clarify subtest output colors

[fahad-munawar]

This PR improves the subtests documentation by clarifying how passed and failed
subtests are displayed when color output is enabled.

The explanatory sentence was moved outside the example output block so it is
not interpreted as terminal output.

Documentation-only change. No functional code changes.

[nicoddemus]

Thanks @fahad-munawar but that section (===================== FAILURES =========) is generated automatically, so it won't look good in the docs.

Also I don't think that is necessary to mention in the docs, to be honest. But thanks anyway!