Yeah documentation on implementing IIO functionality are rather sparse. There is a text file in the kernel sources:
http://lxr.free-electrons.com/source/drivers/staging/iio/Documentation/trigger.txt
What I would recommend is to use at existing implementations (e.g. drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c). However, often that involves to first read about the capabilities and details of the device the that driver is for, then understand that driver code, and finally start implementing the functionality on the actual device at hand…
I think timer interrupts as triggers are possible. There is the Programmable Delay Block, which is kind of a timer. The external trigger capabilities is also documented in the Vybrid Reference Manual (“3.11.1.5 ADC interconnections”).