net/http: extra documentation for Redirect and RedirectHandler

net/http: extra documentation for Redirect and RedirectHandler

Errors with http.Redirect and http.StatusOk seem
to occur from time to time on the irc channel.
This change adds documentation suggesting
to use one of the 3xx codes and not StatusOk
with Redirect.

Change-Id: I6b900a8eb868265fbbb846ee6a53e426d90a727d
Reviewed-on: https://go-review.googlesource.com/15980
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>