API.chm will no longer be generated. However, the other documentation archives have been generated and checked in. The build system should no longer be generating documentation archives for this branch, so these files should be final.
The API.chm file is invalid. Additionally the website API docs are out of date. This is tied to a larger issue with the APIDocTool.
With 4.16, navigate in Windows to Engine\Documentation\CHM.
Result: The API.chm file is only 165 kb
Expected: In 4.15, the file is 94,450 kb
Head over to the existing Questions & Answers thread and let us know what's up.