Why bother? If the people making the thing can't be bothered documenting it you're going to fight an uphill battle. In the long run, for better or for worse, code is documentation. If the code isn't available there'll be only so far you can go. And with the unification of iOS and macOS fast approaching the number of people able to have transparency into even just the POSIX layer is going to drop another order of magnitude.
macOS is dead, Apple just hasn't told anyone about it yet. And iOS 11 is going to be it's murderer.
If you have the time to document, you might as well spend the time documenting Windows, BSD or Linux; Blink or Servo. Or the JS ecosystem.
The Apple specific SMB implementation or APFS or Cocoa are not worth documenting any more than OOXML would be - only as a specification that one party has the power to change and to implement. There is no way for the documentation to be considered prescriptive rather than descriptive for anyone touching the code. It might be useful in some limited aspect for a limited time, but it will decay as Apple considers every single pixel their OS is going to render to be under their control in some way. Apart from what they can sandbox, which amounts to the web platform, which is plenty documented already.
> launchd – second only in importance to the kernel – controlled and orchestrated most of the services supplied in macOS
Who cares about the launch facilities in macOS when anyone who should care about launch facilities is going to be using Windows Server, Linux or BSD? Apple couldn't have been saying fuck off to anyone wanting to screw around with CUPS or Samba or launchd more loudly unless they shouted it from all the rooftops in Cupertino.
> Magic Trackpad 2, never knew how to launch an app from the Dock, or couldn’t manage their Photos library.
1. It'll stop working (here we go, top result for a Google search: https://support.apple.com/en-us/HT204621) 2. Launchpad exists and has a keyboard shortcut and 3. Photos manage themselves. What does manage even mean in the context of you take a picture with a phone, it adds geotagging and a time and that's it. What more do 99% of users want?