Improve SchemaAST documentation and create examples

Currently [Schema.SchemaAST](https://github.com/graph-gophers/graphql-go/blob/v1.5.0/graphql.go#L88) is not well documented. It needs proper documentation with executable examples. For example, we can show how to generate enums or input types in Go (or any other language) for a given schema definition.