SPAM: [RFC PATCH pve-docs] pct: document OCI rootfs replacement semantics

[copystring <copystring@gmail.com>]

Document that replacing an OCI-based container rootfs changes only rootfs, keeps separate mpX mount points, and leaves the previous rootfs as an unused volume for recovery. Also point out that data stored only on / is not carried over.

Signed-off-by: copystring <copystring@gmail.com>
---
 pct.adoc | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/pct.adoc b/pct.adoc
index cd3130f..aded5a9 100644
--- a/pct.adoc
+++ b/pct.adoc
@@ -368,6 +368,39 @@ view of a storage.
 Once the template is on a storage, you can create the container with
 `pct create` or use the wizard in the web interface.
 
+Replacing the Rootfs from an OCI Image
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For an existing OCI-based container, `pct replace-rootfs-from-oci` can replace
+the container's `rootfs` with the contents of another OCI image archive. This is
+an offline operation and requires the container to be stopped.
+
+WARNING: This operation replaces the active `/` filesystem of the container.
+Data that is stored only on the old `rootfs` will not be part of the newly
+active `rootfs`. Separate mount points configured as `mp0`, `mp1`, and so on are
+kept unchanged.
+
+The previous `rootfs` is kept as an `unusedX` volume entry after a successful
+replacement. This allows manual recovery or rollback, but it also means that the
+old volume continues to use storage space until it is explicitly cleaned up.
+
+By default, OCI-derived runtime settings such as the entrypoint, environment,
+OS type and architecture are updated to match the new image. Use
+`--update-oci-config 0` to only switch the `rootfs` while preserving the existing
+runtime settings.
+
+Before replacing the `rootfs`, make sure that all persistent application data is
+stored on separate mount points or backed up. The operation does not try to
+detect application data directories such as `/data` or `/config`.
+
+For example, to replace the `rootfs` of container `999` with a new OCI image
+archive:
+
+----
+# pct replace-rootfs-from-oci 999 local:vztmpl/alpine-3.21.tar \
+    --confirm-rootfs-replace 1
+----
+
 [[pct_settings]]
 Container Settings
 ------------------
-- 
2.55.0.windows.2