Effective Go and the FAQ still had some instances which showed the command line
usage of godoc. Changed them to use go doc.

Updates #25443

Change-Id: If550963322034e6848bc466f79e968e7220e4a88
Reviewed-on: https://go-review.googlesource.com/c/145222Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>