Can't find a doc, but here's a brief list:
6.1
- Support for AP-13x
- Suite-B Encryption
- IPv6 Support for Controller and AP
- Spectrum Analysis using Hybrid APs
- RADIUS Interim Accounting
- RADIUS Server Source Interface Selection
- VPN IKEv2
- Block Traffic Between Clients On the Same Virtual AP
- Device Fingerprinting
- Dashboard Monitoring
6.2
- 7200 controller, RAP-108/109, RAP-3WN/3WNP SUPPORT
- LLDP
- Spectrum Analysis RFPlayback Tool
- Increased AP support for Spectrum Analysis
- DFS support for more APs
- AppRF 1.0 beta (Firewall Visbility)
- Bandwidth contract for RAPs
6.3
- Support for AP-110 and AP-220 series
- LAG support for AP-220 series
- Centralized licensing
- AirGroup
- AppRF 1.0 (firewall visibility)
- HA Fast Failover
- IF-MAP support
- AP Image Preload
- IAP-VPN
Note that some 6.3 features are unavailable on the 6xx series controllers.