publicsuffix: add some commentary on using "go run gen.go".

Also add more detail to some code-gen error messages.

Change-Id: I776609435d3b27d2fffd48046f98fb0b098451c5
Reviewed-on: https://go-review.googlesource.com/19440
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed