Docs for WithNamedContext have a vague reference to the tutorial

Specifically, this line:

https://github.com/haskell-servant/servant/blob/2323906080e50fc2774cd1b43bc59548a90152ed/servant/src/Servant/API/WithNamedContext.hs#L20

I haven't been able to find any information on `WithNamedContext` or `NamedContext` in [the tutorial](https://docs.servant.dev/en/stable/tutorial/index.html), at least using the search bar.