[federation] update @key directive javadoc (#788)

Update Javadoc to clarify repeated directive issue (follow up to #787)

Author: dariuszkuc

graphql-kotlin-federation/src/main/kotlin/com/expediagroup/graphql/federation/directives/KeyDirective.kt

@@ -28,7 +28,8 @@ import graphql.introspection.Introspection.DirectiveLocation
  * well as all the corresponding federated (i.e. extended) types. Key fields specified in the directive field set should correspond to a valid field on the underlying GraphQL interface/object.
  * Federated extended types should also instrument all the referenced key fields with @external directive.
  *
- * NOTE: federation spec specifies that multiple @key directives can be applied on the field which is at odds with graphql-spec and currently unsupported by graphql-kotlin.
+ * NOTE: The Federation spec specifies that multiple @key directives can be applied on the field. The GraphQL spec has been recently changed to allow this behavior,
+ *   but we are currently blocked and are tracking progress in [this issue](https://github.com/ExpediaGroup/graphql-kotlin/issues/590).
  *
  * Example:
  * Given