go/build: update doc.go for go1.3 build tag.

https://code.google.com/p/go/source/browse/src/pkg/go/build/doc.go?r=f15ad332ce135a5b23bfb7d521594afe9fd42f8b
Merge pull request #61 from t-yuki/pkg/go/build/doc

go/build: update doc.go for go1.3 build tag.