Update docs/command output for volume pruning

In previous versions of the Docker API, `system prune --volumes` and `volume prune`
would remove all dangling volumes. With API v1.42, this was changed so that only
anonymous volumes would be removed unless the all filter was specified.

Some of the docs were updated in #4218, however, there were a couple of places
left that didn't make the anonymous vs named volumes distinction clear.

This replaces #4079, which was bitrotted by #4218. See also #4028.

Closes #4079.

Signed-off-by: Ed Morley <501702+edmorley@users.noreply.github.com>

docs/reference/commandline/system_prune.md

@@ -10,7 +10,7 @@ Remove unused data
 | `-a`, `--all`         |          |         | Remove all unused images not just dangling ones    |
 | [`--filter`](#filter) | `filter` |         | Provide filter values (e.g. `label=<key>=<value>`) |
 | `-f`, `--force`       |          |         | Do not prompt for confirmation                     |
-| `--volumes`           |          |         | Prune volumes                                      |
+| `--volumes`           |          |         | Prune anonymous volumes                            |
 
 
 <!---MARKER_GEN_END-->
@@ -50,7 +50,7 @@ Total reclaimed space: 1.84kB
 
 By default, volumes are not removed to prevent important data from being
 deleted if there is currently no container using the volume. Use the `--volumes`
-flag when running the command to prune volumes as well:
+flag when running the command to prune anonymous volumes as well:
 
 ```console
 $ docker system prune -a --volumes
@@ -58,7 +58,7 @@ $ docker system prune -a --volumes
 WARNING! This will remove:
         - all stopped containers
         - all networks not used by at least one container
-        - all volumes not used by at least one container
+        - all anonymous volumes not used by at least one container
         - all images without at least one container associated to them
         - all build cache
 Are you sure you want to continue? [y/N] y