Update K3s upstream developer docs

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

developer-docs/k3s.md

@@ -11,6 +11,7 @@ Until then, here is a handy table:
 |-------------------------------------------------------------|-----------------------------------------------------------|---|
 | [release-1.18](https://github.com/rancher/rke2/tree/release-1.18) | [release-1.19](https://github.com/k3s-io/k3s/tree/release-1.19) | Making k3s an embeddable engine required changes developed after release-1.18 was branched. |
 | [release-1.19](https://github.com/rancher/rke2/tree/release-1.19) | [release-1.19](https://github.com/k3s-io/k3s/tree/release-1.19) | RKE2 development stayed on 1.18 for a long time, essentially jumping from 1.18 to 1.20 with both release-1.18 and release-1.19 forked off master close to each other. |
-| [release-1.20](https://github.com/rancher/rke2/tree/release-1.20) | [master](https://github.com/k3s-io/k3s/tree/master) | Rolling commit from k3s master; should be moved to release-1.21 |
-| [release-1.21](https://github.com/rancher/rke2/tree/release-1.21) | [master](https://github.com/k3s-io/k3s/tree/master) | Rolling commit from k3s master; should be moved to release-1.21 |
+| [release-1.20](https://github.com/rancher/rke2/tree/release-1.20) | [engine-1.21](https://github.com/k3s-io/k3s/tree/engine-1.21) | The K3s engine-1.21 branch was forked from K3s master just before master was updated to Kubernetes 1.22, and contains critical changes necessary to support RKE2 on Windows. |
+| [release-1.21](https://github.com/rancher/rke2/tree/release-1.21) | [engine-1.21](https://github.com/k3s-io/k3s/tree/engine-1.21) | Same K3s upstream as the RKE2 release-1.20 branch. |
+| [release-1.22](https://github.com/rancher/rke2/tree/release-1.22) | [engine-1.21](https://github.com/k3s-io/k3s/tree/release-1.22) | We plan to better align the K3s and RKE2 release-1.22 branches, when they are forked. |
 | [master](https://github.com/rancher/rke2/tree/master) | [master](https://github.com/k3s-io/k3s/tree/master) | Rolling commit from K3s master |