Speaking as the person who re-wrote the majority of ehci debug port code, what further documentation are you interested in?
It does appear we might be missing an addendum to the documentation to explain about all the different uses of the USB debug port dongle. In recent kernels you can use a USB debug port for early_printk, kdb, kgdb, or even a system level serial console, which has support for sysrq. There is a mention of the configuration arguments in the kernel-parameters.txt, but that doesn't cover anything about the useful ways to use the device.
The document in the kernel which describes the device definitely doesn't seem to elaborate on all that you can do with the device.