improve operation with client directive usage documentation

[n1ru4l]

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.

[greenreign]

Yeah. Also, how do you do that?

[andoks]

Related to #2736 - it would be nice if the docs mentioned that alternative graphql servers may have alternative graphql root node names.

Also related documentation here: https://the-guild.dev/graphql/codegen/docs/integrations/apollo-local-state