cmd/godoc: better error message for missing index files

Fixes issue 5024.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8222045
testing: document that Log and Logf do not usually produce output
The text is printed only if the test fails or -test.v is set.
Document this behavior in the testing package and 'go help test'.
Also put a 'go install' into mkdoc.sh so I don't get tricked by the
process of updating the documentation ever again.

Fixes issue 5174.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/8118047