Only documentation / comment changes. Update references to
point to golang.org permalinks or go.googlesource.com/go.
References in historical release notes under doc are left as is.

Change-Id: Icfc14e4998723e2c2d48f9877a91c5abef6794ea
Reviewed-on: https://go-review.googlesource.com/4060
Reviewed-by: Ian Lance Taylor <iant@golang.org>

We are not the right people to support editor plugins, and the profusion
of editors in this CL demonstrates the unreality of pretending to do so.
People are free to create and advertise their own repos with support.

For discussion: https://groups.google.com/forum/#!topic/golang-dev/SA7fD470FxU

LGTM=rminnich, kamil.kisiel, gri, rsc, dave, josharian, ruiu
R=golang-codereviews, rminnich, kamil.kisiel, gri, rsc, dominik.honnef, dave, josharian, ruiu, ajstarks
CC=golang-codereviews
https://golang.org/cl/105470043