clarify which context key is used

[adamdecaf]

What did you do?

The current documentation states:

// The HTTP client to use is derived from the context.
// If a client is not provided via the context, http.DefaultClient is used.

What did you expect to see?

There's no clear indication how one is to alter the Context in order to mutate that client (i.e. setting timeouts). We know that the default timeouts are bad (#24138) and so modifying those is important to do.

[adamdecaf]

I created https://golang.org/cl/134436