cmd/godoc: ignore misnamed examples and print a warning

Fixes issue 4211.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/6970051

Committer: Andrew Gerrand <adg@golang.org>
A+C: Kamil Kisiel (individual CLA)

Generated by addca.

R=gobot
CC=golang-dev
https://codereview.appspot.com/7038046
doc: update standard copyright header

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/7034044
doc: add race detector manual

R=minux.ma, franciscossouza, rsc, adg, adg
CC=golang-dev
https://codereview.appspot.com/6948043