Add API groups explanation to golang tutorial (#5842)

* Add API groups explanation to golang tutorial
* Add blurb about API groups being used for other things internally to golang tutorial

Signed-off-by: Jonathan Berkhahn <[email protected]>

Author: jberkhahn

website/content/en/docs/building-operators/golang/tutorial.md

@@ -37,6 +37,12 @@ cd $HOME/projects/memcached-operator
 # so all API groups will be <group>.example.com
 operator-sdk init --domain example.com --repo github.com/example/memcached-operator
 ```
+`--domain` will be used as the prefix of the API group your custom resources will be created in.
+API groups are a mechanism to group portions of the Kubernetes API. You're probably already familiar with
+some of the core Kubernetes API groups, such as `apps` or `rbac.authorization.k8s.io`. API groups are used
+internally to version your Kubernetes resources and are thus used for many things. Importantly, you should 
+name your domain to group your resource types in meaningful group(s) for ease of understanding and because these
+groups determine how access can be controlled to your resource types using RBAC. For more information, see [the core Kubernetes docs](https://kubernetes.io/docs/reference/using-api/#api-groups) and [the Kubebuilder docs](https://book.kubebuilder.io/cronjob-tutorial/gvks.html).
 
 **Note** If your local environment is Apple Silicon (`darwin/arm64`) use the `go/v4-alpha`
 plugin which provides support for this platform by adding to the init subCommand the flag `--plugins=go/v4-alpha`