docs: explain nondeterministic row ordering without ORDER BY (#7373)

[Xavrir]

Summary

• document that DataFusion only guarantees row order when a query uses ORDER BY
• add notes to the SQL SELECT guide for joins, GROUP BY, and the ORDER BY clause itself
• add an FAQ entry for users who see the same rows come back in a different order between runs

Testing

• docs-only change; no code paths changed

Closes #7373