FieldConfigArgument descriptions are not used in GraphiQL

Given the following configuration:
```go
Args: relay.NewConnectionArgs(graphql.FieldConfigArgument{
"fromDate": &graphql.ArgumentConfig{
Description: "From date value with `YY-MM-DD` format",
Type: graphql.DateTime,
},
"toDate": &graphql.ArgumentConfig{
Description: "To date value with `YY-MM-DD` format",
Type: graphql.DateTime,
},
"status": &graphql.ArgumentConfig{
Description: "Current invoice status retrieved from transactions",
Type: invoiceTransactionStatusEnum,
},
}),
```
The custom descriptions are not passed to **GraphiQL**.
For example, the type graphql.DateTime has its default description that is not overrided from my custom description. Indeed, they keep the default description:
`The DateTime scalar type represents a DateTime. The DateTime is serialized as an RFC 3339 quoted string`

Instead, the field `status` which is a custom enumeration, comes with `No Description`.