I have read that limits may be programmed for the output current of GPIO pins. However, after reading several online versions of Broadcom's data sheet for the SoC, I've found nothing to corroborate the data in this reference. Is there a "definitive source" for this GPIO current limit function, either in hardware documentation OR software documentation?
Yes. The definitive source for RPi hardware documentation is the Raspberry Pi Foundation. The Foundation updated their published documentation to include programmable GPIO current limits recently (Jan, 2019), and the latest "official documentation" of the GPIO docs can be found here.
In general, The Foundation's "official documentation" is published in the following locations:
The Foundation has also referenced some documents published by Broadcom as part of their official documentation. Specifically with respect to the GPIO, you may wish to consult this document beginning on p. 89.