Documentation on Apple's USB Chargers?
The "chargers" are just 5V switchmode power supplies. 

They have a bit of a trick (resistors on the otherwise unused USB data lines) that allows the unit plugged into the supply to know the current capability, but the charging circuitry itself is built into the device with the battery. 

If you're concerned about drawing excessive current from the supply, you could limit yourself to the smallest charger's capability, detect the Apple resistors, or implement a more general solution. This chip may be of interest: MAX14617, which emulates adapters (including Apple adapters) and thus has the information required to specify the design of circuitry to detect the adapter emulation. 

Here is some hacker-level reverse engineering that should give you the idea. This limits an iPhone to 0.5A charging: