docs: add note about systemd df RECLAIMABLE bug

Our calculation is just wrong and the way the entire API is designed it
cannot work. This is the same interface as docker is using and they have
the same bug there. So simply document this as known problem, in case
users complain we at least have something to point to.

An actual fix might be possible but not without reworking the full API
and because this is exposed in the docker compat and libpod REST API we
cannot really change it.

Signed-off-by: Paul Holzinger <[email protected]>

Author: Luap99

Diff for: docs/source/markdown/podman-system-df.1.md

@@ -7,7 +7,11 @@ podman\-system\-df - Show podman disk usage
 **podman system df** [*options*]
 
 ## DESCRIPTION
-Show podman disk usage
+Show podman disk usage for images, containers and volumes.
+
+Note: The RECLAIMABLE size that is reported for images can be incorrect. It might
+report that it can reclaim more than a prune would actually free. This will happen
+if you are using different images that share some layers.
 
 ## OPTIONS
 #### **--format**=*format*