The short guide shows developers how to set up their development environment, install the Amazon Appstore and handle input compatibility considerations for Windows devices involving keyboards, mice and window management.
The new documentation says that the Windows Subsystem for Android "is currently only available through preview in the Windows Insiders Program." However, as of right now, the Windows Subsystem for Android code isn't in any external Insider test builds, including the Dev Channel. My guess is this will happen any day now, given the publication of the documentation.
Update: Here's more from Microsoft's blog post about the first preview of the Windows Subsystem for Android. The subsystem is designed to make it so the curated list of Android apps and games can run side-by-side using the Snap Layouts feature, pinned to the Start menu or Taskbar and/or integrated into Alt+ Tab and Task view. Notifications from the Android apps notifications will show up in the Action Center and be able to be shared using a clipboard between a Windows app and an Android one.
The Windows Subsystem for Android will be an app available from the Microsoft Store. The Android subsystem app runs in a Hyper-V virtual machine, just like the Windows Subsystem for Linux.