docs: clarify partial vs lazy image pulling

The performance tutorial calls Podman's zstd:chunked / eStargz support
"lazy pulling", which isn't accurate. By itself Podman only does partial
pulling: it skips layer content it already has, but the image is still
fully pulled before a container runs. Real lazy pulling, where the image
is mounted before the download finishes, needs an external snapshotter
such as stargz-snapshotter.

Rename the section to "partial pulling" and add a sentence spelling out
the difference, as @giuseppe suggested on the issue.

Fixes: #24947

Signed-off-by: Grzegorz Szczepanczyk <g.szczepanczyk@getprintbox.com>

Author: stefan8

docs/tutorials/performance.md

@@ -180,21 +180,23 @@ $ podman info -f '{{.Host.RootlessNetworkCmd}}'
 pasta
 ```
 
-### Lazy pulling of container images
+### Partial pulling of container images
 
-Podman supports lazy pulling for the following container image formats:
+Podman supports partial pulling for the following container image formats:
 
 * __zstd:chunked__
 
 * __eStargz__
 
 __zstd:chunked__ has better performance than __eStargz__.
 
+Partial pulling downloads only the layer contents that are missing locally, but the image is still fully pulled before its containers run. Lazy pulling, where the image is mounted before it is fully downloaded, is out of scope for Podman itself and requires an additional snapshotter such as the [stargz-snapshotter](https://github.com/containerd/stargz-snapshotter) configured as an additional layer store.
+
 See the article [_Pull container images faster with partial pulls_](https://www.redhat.com/sysadmin/faster-container-image-pulls) by Giuseppe Scrivano and Dan Walsh.
 
 ### Choosing a host file system
 
-Lazy pulling of container images can run more efficiently when the file system has reflink support. The file systems XFS and BTRFS have reflink support.
+Partial pulling of container images can run more efficiently when the file system has reflink support. The file systems XFS and BTRFS have reflink support.
 
 ### Option --log-driver