docs(package): describe snapshot view configuration

Document the snapshotView annotation in both the package overview and
rootfs guide.

Explain how to enable the feature from config.json or urunc.json and
clarify that it is used together with mountRootfs on supported
block-based snapshotters.

Signed-off-by: sidneychang <2190206983@qq.com>

Author: sidneychang

docs/package/index.md

@@ -72,6 +72,12 @@ Except of the above, `urunc` accepts the following optional annotations:
 - `com.urunc.unikernel.mountRootfs`: A boolean value that if it is `true`,
   requests from `urunc` to mount the container's image rootfs in the unikernel
   (either as a block device or through shared-fs).
+- `com.urunc.unikernel.snapshotView`: A boolean value that if it is `true`,
+  enables the per-container snapshot-view feature. It is **not** stored in the
+  OCI image at build time; it is supplied at container start as a runtime
+  annotation. This feature is used only together with
+  `com.urunc.unikernel.mountRootfs=true` and currently targets block-based
+  snapshotters such as `devmapper` and `blockfile`.
 
 Due to the fact that [Docker](https://www.docker.com/) and some high-level
 container runtimes do not pass the image annotations to the underlying container
@@ -80,6 +86,18 @@ container's rootfs. The file should be named `urunc.json`, it should be
 placed in the root directory of the container's rootfs and it should have a JSON
 format with the above information, where the values are base64 encoded.
 
+The recommended way to enable the snapshot-view feature is through OCI runtime
+annotations at container start time:
+
+- Add `com.urunc.unikernel.snapshotView=true` to the OCI runtime annotations in
+  the container bundle's `config.json`. This does not require rebuilding the
+  image and keeps snapshot-view opt-in per run.
+
+At the moment `snapshotView` is consumed from the OCI runtime annotations in
+the bundle's `config.json`. The `mountRootfs` setting may still come from the
+bundle annotation, container metadata, or `urunc.json`, but `snapshotView`
+itself is expected to be passed at runtime.
+
 ## Tools to construct OCI images with `urunc`'s annotations
 
 As previously mentioned we currently provide 2 different tools to build and