Check this document for information on the new option for the WAN port: draft-nat-port-mapping.txt
Basically, it allows clients to create their own temporary port forwarding mappings and also allows the Base Station to advertise those mappings to the world. Enabling this option is a potential security issue so yes, a KB article from Apple regarding the ramifications of this new wrinkle would be much appreciated, not to mention an update to the Designing Airport Networks manual. For now, a rule of thumb would be: If you don't what it does and the ABS does what you need as-is, then don't enable it.