Document/stabilise boot.initPath and friends

[K900]

Is your feature request related to a problem? Please describe.
We currently have to resort to symlinking wrapper scripts into well-known locations to make NixOS run on WSL, because we don't generally install things to "normal" locations. However, there are undocumented wsl.conf options that allow us to directly specify the right paths in the config...

Describe the solution you'd like
boot.initPath, boot.initShutdownCommand and boot.initWaitCommand wsl.conf settings are stabilized and made part of public API.

Describe alternatives you've considered
Continue doing what we're doing, which is tolerable but prone to breaking should WSL ever change how it does things.

Additional context
I found the settings by accident while poking the init binary with strings. I don't know if they're a recent addition or not.