Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Improve explanation of ReplicaSet.

"ReplicaSet represents the configuration of a ReplicaSet" does not tell
you much. I took the explanation from
https://kubernetes.io/docs/concepts/workloads/controllers/replicaset/#when-to-use-a-replicaset



**What this PR does / why we need it**:

`kubectl explain rs` prints "ReplicaSet represents the configuration of a ReplicaSet", which is not very helpful.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
Improve explanation of ReplicaSet
```