docs: clarify independence of Night-Humidity-Priority, Device Dampening, and Ambient Control

Updated documentation to clearly explain that the three control features are completely independent:

- Night-Humidity-Priority (nightVPDHold): Only active when nightVPDHold=TRUE and lights OFF
- VPD Device Dampening (vpdDeviceDampening): Only active when vpdDeviceDampening=TRUE
- Ambient Control (ambientControl): Only active when ambientControl=TRUE

Key clarifications:
- Methods are ALWAYS called for proper integration in the action pipeline
- But the logic inside methods ONLY executes when their respective setting is TRUE
- No cross-dependencies - each feature only checks its own setting
- All three can be enabled/disabled independently by the user

Updated files:
- docs/action_flow.md: Added Feature Independence section, clarified Step 1-3
- docs/core_concepts/CONTROL_MODES.md: Updated Night VPD Hold integration section

Author: Luca Azzinnaro