Clarify potential security issues; use safer example config

[colinodell]

Description:

This PR has two goals:

1. Clarify exactly what can happen if use_x_forwarded_for is enabled
2. Update the example configuration to be safer in case somebody blindly copy-pastes it

See home-assistant/core#14345 (comment) for why a misconfigured setting can be a problem.

Checklist:

• Branch: Fixes, changes and adjustments should be created against current. New documentation for platforms/components and features should go to next.
• The documentation follow the standards.

[mnoorenberghe]

LGTM

[fabaff]

Thanks 🐦