net/http: document Client's new header copying behavior

[bradfitz]

The http Client's new copy-headers-on-redirect behavior is not documented.

It should probably be added to https://tip.golang.org/pkg/net/http/#Client.Do and/or https://tip.golang.org/pkg/net/http/#Client with maybe a pointer to its canonical documentation location from the CheckRedirect field.

@dsnet, can you write this? Otherwise reassign to me if you're swamped.

[bradfitz]

And then update go1.8.html which now has a reference to this bug.

[gopherbot]

CL https://golang.org/cl/33670 mentions this issue.