There is. You may just not have understood that this sentence: “The pin is internally connected to the VBUS of the USB port.” , implies exactly that.
Implies means you have to guess, which is extremely dangerous with a datasheet. The datasheet also explicitly says you can connect up to 6.5V on V(LiPo), which thanks to the schematic, we know is a "release the magic smoke" class of mistake. I don't think guessing what the datasheet is implying is a good idea with this document.
The schematic is helpful, but it's not the most easy to read, it would would if the pins were more clearly marked for example. To say "it's in the schematic so you should know" is not fair to the average user, especially when it contradicts the text of the datasheet.
@dkryder this ties in closely with the subject of this thread. The absolute maximum for the battery pin is 4.9V and the recommended maximum is 4.5V. 4 NiMH cells (nominally 1.2V each) will take you above that 4.9 and likely damage the Xenon. So you have to use 3 NiMH cells. The minimum voltage is 3.3V and 3.4-3.5 is better for stability. NiMH cells do an excellent job holding their terminal voltage as they drain, but that's still probably asking too much of them.
You may want to consider 4 cells connected to the USB power input, but you'll have to find out of that has any implications on power drain. Also don't charge the NiMH using the onboard LiPo charging circuit.
I think it's pretty bad that Particle isn't willing to say anything or release any documentation on powering their devices with standard batteries. This would be a much more common usage case for most people than LiPos. Considering how good, cheap,and convenient NiMH's are, I can't imagine ever using LiPos, which cost more than the current price of a Xenon around here anyway.