docs: update du cmd description

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

docs/reference/buildx_du.md

@@ -20,6 +20,70 @@ Disk usage
 
 ## Examples
 
+### Show disk usage
+
+The `docker buildx du` command shows the disk usage for the currently selected
+builder.
+
+```console
+$ docker buildx du
+ID                                RECLAIMABLE    SIZE          LAST ACCESSED
+12wgll9os87pazzft8lt0yztp*        true           1.704GB       13 days ago
+iupsv3it5ubh92aweb7c1wojc*        true           1.297GB       36 minutes ago
+ek4ve8h4obyv5kld6vicmtqyn         true           811.7MB       13 days ago
+isovrfnmkelzhtdx942w9vjcb*        true           811.7MB       13 days ago
+0jty7mjrndi1yo7xkv1baralh         true           810.5MB       13 days ago
+jyzkefmsysqiaakgwmjgxjpcz*        true           810.5MB       13 days ago
+z8w1y95jn93gvj92jtaj6uhwk         true           318MB         2 weeks ago
+rz2zgfcwlfxsxd7d41w2sz2tt         true           8.224kB*      43 hours ago
+n5bkzpewmk2eiu6hn9tzx18jd         true           8.224kB*      43 hours ago
+ao94g6vtbzdl6k5zgdmrmnwpt         true           8.224kB*      43 hours ago
+2pyjep7njm0wh39vcingxb97i         true           8.224kB*      43 hours ago
+Shared:        115.5MB
+Private:       10.25GB
+Reclaimable:   10.36GB
+Total:         10.36GB
+```
+
+If `RECLAIMABLE` is false, the `docker buildx du prune` command won't delete
+the record, even if you use `--all`. That's because the record is actively in
+use by some component of the builder.
+
+The asterisks (\*) in the output indicate the following:
+
+- An asterisk next to an ID (`zu7m6evdpebh5h8kfkpw9dlf2*`) indicates that the record
+  is mutable. The size of the record may change, or another build can take ownership of
+  it and change or commit to it. If you run the `du` command again, this item may
+  not be there anymore, or the size might be different.
+- An asterisk next to a size (`8.288kB*`) indicates that the record is shared.
+  Storage of the record is shared with some other resource, typically an image.
+  If you prune such a record then you will lose build cache but only metadata
+  will be deleted as the image still needs to actual storage layers.
+
 ### <a name="builder"></a> Override the configured builder instance (--builder)
 
-Same as [`buildx --builder`](buildx.md#builder).
+Use the `--builder` flag to inspect the disk usage of a particular builder.
+
+```console
+$ docker buildx du --builder youthful_shtern
+ID                                RECLAIMABLE    SIZE          LAST ACCESSED
+g41agepgdczekxg2mtw0dujsv*        true           1.312GB       47 hours ago
+e6ycrsa0bn9akigqgzu0sc6kr         true           318MB         47 hours ago
+our9zg4ndly65ze1ccczdksiz         true           204.9MB       45 hours ago
+b7xv3xpxnwupc81tc9ya3mgq6*        true           120.6MB       47 hours ago
+zihgye15ss6vum3wmck9egdoy*        true           79.81MB       2 days ago
+aaydharssv1ug98yhuwclkfrh*        true           79.81MB       2 days ago
+ta1r4vmnjug5dhub76as4kkol*        true           74.51MB       47 hours ago
+murma9f83j9h8miifbq68udjf*        true           74.51MB       47 hours ago
+47f961866a49g5y8myz80ixw1*        true           74.51MB       47 hours ago
+tzh99xtzlaf6txllh3cobag8t         true           74.49MB       47 hours ago
+ld6laoeuo1kwapysu6afwqybl*        true           59.89MB       47 hours ago
+yitxizi5kaplpyomqpos2cryp*        true           59.83MB       47 hours ago
+iy8aa4b7qjn0qmy9wiga9cj8w         true           33.65MB       47 hours ago
+mci7okeijyp8aqqk16j80dy09         true           19.86MB       47 hours ago
+lqvj091he652slxdla4wom3pz         true           14.08MB       47 hours ago
+fkt31oiv793nd26h42llsjcw7*        true           11.87MB       2 days ago
+uj802yxtvkcjysnjb4kgwvn2v         true           11.68MB       45 hours ago
+Reclaimable:    2.627GB
+Total:          2.627GB
+```