Because the thing you are looking for is likely not in that repo. Read the "Readme.md" in the api directory:
This repository contains .NET Documentation. Contribute to dotnet/docs development by creating an account on GitHub.
github.com
The "conceptual" documentation is in that repo. The other repo referred to in the read me, points to the documentation for most of the APIs which seems to be what you are after. Unfortunately, most of that documentation will be in .XML rather than MarkDown. You'll need to find a tool that converts that XML into something like a CHM or HTML document. (Yes they exist, you just need to search for such tools.)
I think that
@cjard pointed you to the conceptual documentation because that is the documentation that makes more sense to have around as PDFs or some other formats to read offline. Do you really want to be reading about LinkedList<T> while offline? What makes more sense to read offline are the various how-to and architectural documentation.