`go vet` will complain when an exported method is not properly
documented. This change adds comments to the generated `MarshalJSON`,
`UnmarshalJSON`, `MarshalYAML`, and `UnmarshalYAML` methods to satisfy
`go vet`.
As of go 1.10, the go test command now automatically runs go vet on the
package being tested. Complaints from `go vet` are treated like build
errors and prevent execution of the test. This means that generated code
that is not properly documented will prevent test execution.
[1]: https://golang.org/doc/go1.10#test-vet