net/http: document more that Get/Post/RoundTrip response is streaming

[bradfitz]

Document on the http.Response type at least that Responses in Go are streaming, and the Response just represents the response headers, but the Body may not have arrived yet, and may fail while it's read.

If we put this on the Response type, we don't need to duplicate it in a dozen places.

(Suggestion by @fanirthuban in #22235 (comment))

[gopherbot]

Change https://golang.org/cl/80075 mentions this issue: net/http: document streaming nature of Response.Body