This is problem with vs files that are generated on fresh project launch, on projects with old VS files everything works as expected

Steps to reproduce:
1. Open Unity version 2018.1.0b2 or higher
2. Create new project
3. Create new script and open it from Unity (make sure VS2017 is selected as "External Script Editor")
4. Click on help option in Visual Studio menu bar and see that there is missing "Unity API Reference" option

Expected result: There should be "Unity API Reference" option in all unity project scripts opened with Visual Studio.

Note: launching older project with older VS generated files on 2018.1.0b2 and up the API Reference option will appear as expected in Visual Studio

Didn't reproduce with: 2018.1.0a1, 2018.1.0a3 , 2018.1.0a5, 2018.1.0b1
Reproduced with: 2018.1.0b2, 2018.1.0b3
Regressed in 2018.1.0b2