The Go language has a wonderfully comprehensive standard library. There is documentation for all of it. You can access that documentation anytime if you have an internet connection via https://golang.org/doc/.
If you are without an internet connection, you're still in luck. Go has a built-in feature for serving the documentation locally offline. Just run the following command:
$ godoc -http=:6060
and then visit localhost:6060.
Note: if you do not already have godoc installed, you can install it with:
$ go install golang.org/x/tools/cmd/godoc@latest