The C# Language Specification is the definitive source for C# syntax and usage. This specification contains detailed information about all aspects of the language, including many points that the documentation for Visual C# doesn't cover.
You can download version 5.0 of this specification from the Microsoft Download Center. If you've installed Visual Studio 2015, you can also find the specification on your computer in the Program Files (x86)/Microsoft Visual Studio 14.0/VC#/Specifications/1033 folder. If you have another version of Visual Studio installed or if you installed Visual Studio in a language other than English, change the path as appropriate.
Version 6.0 of the specification has not been approved as a standard. This site contains the draft C# 6.0 specification. It is built from the markdown files contained in the dotnet/csharplang GitHub repository.