net/http: clean up the Client redirect code, document Body.Close rules more
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.