Issue #8633 (and #9134) noted that we didn't document the rules about
closing the Response.Body when Client.Do returned both a non-nil
*Response and a non-nil error (which can only happen when the user's
CheckRedirect returns an error).

In the process of investigating, I cleaned this code up a bunch, but
no user-visible behavior should have changed, except perhaps some
better error messages in some cases.

It turns out it's always been the case that when a CheckRedirect error
occurs, the Response.Body is already closed. Document that.

And the new code makes that more obvious too.

Fixes #8633

Change-Id: Ibc40cc786ad7fc4e0cf470d66bb559c3b931684d
Reviewed-on: https://go-review.googlesource.com/21364
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>