All other exported errors in net/http are commented.  This change adds
documentation to ErrNoCookie and ErrNoLocation to explain where they are
returned, and why.

Change-Id: I21fa0d070dd35256681ad0714000f238477d4af1
Reviewed-on: https://go-review.googlesource.com/11044Reviewed-by: Russ Cox <rsc@golang.org>