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>

cli/command/volume/prune.go

@@ -27,7 +27,7 @@ func NewPruneCommand(dockerCli command.Cli) *cobra.Command {
 
 	cmd := &cobra.Command{
 		Use:   "prune [OPTIONS]",
-		Short: "Remove all unused local volumes",
+		Short: "Remove unused local volumes",
 		Args:  cli.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			spaceReclaimed, output, err := runPrune(dockerCli, options)