Yup, the docs lag behind a few years here 
The list of availabe interrupt pins for the Electron is similar to the Photon’s.
To know which pins share EXTI lines with other pins you’d need to look at the GPIO numbers in the pin overview in the datasheets where Px## stands for Port x bit ## and all bits with the same number share the same EXTI line across all ports.
So out of Ports A, B, C & D only one ## can have an independent interrupt assigned.
e.g. for the Photon you read D0 and A5 can’t be used since they share the EXTI line with SETUP - that’s because D0 is PB 7, A5 is PA 7 and SETUP is PC 7 (see https://docs.particle.io/datasheets/photon-datasheet/#pinout-diagrams)
For the Electron the GPIO setup can be found here (unfortunately the extra GPIOs for SETUP & RGB are not documented there as they are for the Photon)
(@KyleG, could you get someone at Particle to update the docs accordingly, please)