context: relax recommendation against putting Contexts in structs

This is a follow-on from discussion in #14660.

Right now the context package documentation says

Do not store Contexts inside a struct type; instead, pass a Context explicitly to each function that needs it. The Context should be the first parameter, typically named ctx: [...]

This advice seems overly restrictive. @bradfitz wrote in that issue:

While we've told people not to add contexts to structs, I think that guidance is over-aggressive. The real advice is not to store contexts. They should be passed along like parameters. But if the struct is essentially just a parameter, it's okay. I think this concern can be addressed with package-level documentation and examples.

Let's address this concern with package-level documentation.

Also see @rsc's comment at https://github.com/golang/go/issues/14660#issuecomment-200145301.

/cc @Sajmani @bradfitz @rsc