Follow-up for cherry-picked PR #8272:
- Add MATRIX_RECOVERY_KEY to module docstring header in matrix.py
- Register in OPTIONAL_ENV_VARS (config.py) with password=True, advanced=True
- Add to _NON_SETUP_ENV_VARS set
- Document cross-signing verification in matrix.md E2EE section
- Update migration guide with recovery key step (step 3)
- Add to environment-variables.md reference