chore(6.x.x): update docs (#2085)

Co-authored-by: Samuel Vazquez <[email protected]>

Author: samuelAndalon

website/versioned_docs/version-6.x.x/schema-generator/execution/fetching-data.md

@@ -55,3 +55,19 @@ You can provide your own custom data fetchers to resolve functions and propertie
 to your [SchemaGeneratorConfig](https://github.com/ExpediaGroup/graphql-kotlin/blob/master/generator/graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/SchemaGeneratorConfig.kt).
 
 See our [spring example app](https://github.com/ExpediaGroup/graphql-kotlin/tree/master/examples/server/spring-server) for an example of `CustomDataFetcherFactoryProvider`.
+
+:::info
+
+Currently, graphql-kotlin, through the [KotlinDataFetcherFactoryProvider](https://github.com/ExpediaGroup/graphql-kotlin/blob/master/generator/graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/execution/KotlinDataFetcherFactoryProvider.kt#L62)
+creates a [PropertyDataFetcher](https://github.com/ExpediaGroup/graphql-kotlin/blob/master/generator/graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/execution/PropertyDataFetcher.kt)
+per source's property. This instance is created [every single time](https://github.com/graphql-java/graphql-java/blob/master/src/main/java/graphql/schema/GraphQLCodeRegistry.java#L100)
+the graphql-java [DataFetcherFactory](https://github.com/graphql-java/graphql-java/blob/master/src/main/java/graphql/schema/DataFetcherFactory.java) is invoked,
+which happens to be on runtime per property per GraphQL operation.
+
+If you want to avoid that, use or extend the [SimpleSingletonKotlinDataFetcherFactoryProvider](https://github.com/ExpediaGroup/graphql-kotlin/blob/master/generator/graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/execution/KotlinDataFetcherFactoryProvider.kt#L72) which will provide a
+[SingletonPropertyDataFetcher](https://github.com/ExpediaGroup/graphql-kotlin/blob/master/generator/graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/execution/SingletonPropertyDataFetcher.kt) that will host its own singleton factory, and it will store
+all `KProperty.Getter<*>`s in a `ConcurrentHashMap`.
+
+This is inspired by this [graphql-java's PR](https://github.com/graphql-java/graphql-java/pull/3754)
+
+:::