To repro:
0. Make sure your system is clean (no Unity or VS were installed on it before).
1. Using download assistant install Unity (5.3.1 and higher) + Visual Studio 2015 + Visual Studio Unity Tools.
2. Open Unity - Create a new project - Create new script - Open it in Visual Studio 2015.
3. Select "Monobehaviour" in script and Open Help - Unity API Reference (or just Open Help - Unity API Reference).
4. Search in documentation is opened.
5. Open Help - Unity API Reference.

Actual result: Unity API Reference Is grey and there is no ability to open it again.
Expected results: Unity API Reference can be used in any case, from Help dropdown or using shortcuts.