doc: explain why certain features are unsupported

Signed-off-by: Flemming Frandsen <dren.dk@gmail.com>

docs/deploy-rbd.md

@@ -55,7 +55,7 @@ make image-cephcsi
 | `dataPool`                                                                                          | no                   | Ceph pool used for the data of the RBD images.                                                                                                                                                                          |
 | `volumeNamePrefix`                                                                                  | no                   | Prefix to use for naming RBD images (defaults to `csi-vol-`).                                                                                                                                                           |
 | `snapshotNamePrefix`                                                                                | no                   | Prefix to use for naming RBD snapshot images (defaults to `csi-snap-`).                                                                                                                                                 |
-| `imageFeatures`                                                                                     | yes                  | RBD image features. CSI RBD currently supports only `layering` feature. See [man pages](http://docs.ceph.com/docs/master/man/8/rbd/#cmdoption-rbd-image-feature)                                                        |
+| `imageFeatures`                                                                                     | yes                  | RBD image features. CSI RBD currently supports only `layering` feature. See [man pages](http://docs.ceph.com/docs/master/man/8/rbd/#cmdoption-rbd-image-feature) Note that the required support for [object-map and fast-diff were added in 5.3 and journaling does not have KRBD support yet](https://docs.ceph.com/en/latest/rbd/rbd-config-ref/#image-features) which is why they are unsupported here. deep-flatten is added for cloned images. |
 | `mapOptions`                                                                                        | no                   | Map options to use when mapping rbd image. See [krbd](https://docs.ceph.com/docs/master/man/8/rbd/#kernel-rbd-krbd-options) and [nbd](https://docs.ceph.com/docs/master/man/8/rbd-nbd/#options) options.                |
 | `unmapOptions`                                                                                      | no                   | Unmap options to use when unmapping rbd image. See [krbd](https://docs.ceph.com/docs/master/man/8/rbd/#kernel-rbd-krbd-options) and [nbd](https://docs.ceph.com/docs/master/man/8/rbd-nbd/#options) options.            |
 | `csi.storage.k8s.io/provisioner-secret-name`, `csi.storage.k8s.io/node-stage-secret-name`           | yes (for Kubernetes) | name of the Kubernetes Secret object containing Ceph client credentials. Both parameters should have the same value                                                                                                     |