It is not true that the whole device is open source.
Regarding documentation on the Broadcom chip there are three questions on the official Raspberry Pi FAQ that address this point directly.
What hardware documentation will be available?
Broadcom don’t release a full datasheet for the BCM2835, which is the chip at the heart of the Raspberry Pi. We will release a datasheet for the SoC which will cover the hardware exposed on the Raspi board e.g. the GPIOs. We will also release a board schematic later on.
But I want documentation for [hardware X]!
Other documentation may be released in future but this will be at the Foundation’s discretion.
But I demand the documentation for the chip. Give it to me!
To get the full SoC documentation you would need to sign an NDA with Broadcom, who make the chip and sell it to us. But you would also need to provide a business model and estimate of how many chips you are going to sell.
The schematics for the boards were posted to the Raspberry Pi blog on the 19th of April 2012.