improve operation with client directive usage documentation

We have documentation for usage with multiple schemas: https://www.graphql-code-generator.com/docs/config-reference/schema-field#multiple-schemas-and-client-side-schema

But we should also add a small section on adding definitions for client SDL directives such as `directive @client on FIELD`, for people that are using those (e.g. via apollo-client) with an example, so we can point crisp users to that.