Reproduction steps:

1. Create a new Project
2. Create a new C# Script
3. Open it with a Code Editor (VS)
4. Type "GameObject" and see the documentation of that class
5. In the "Start()" function type "List" and see that there is no documentation for that class

Expected Result: Every class should have its own documentation
Actual Result: Only Unity specific classes have their own documentation

Reproduced with: 2018.1.0a7, 2017.3.0f1, 2017.2.0p4, 2017.1.2p3, 5.6.4p3

Note:
Visual Studio Community 2017 Version 15.3.3