fix(volume/prune): document label!= filter in --filter flag description

The --filter flag for 'docker volume prune' previously only showed
'label=<label>' as an example, not mentioning the negation form
'label!=<label>', even though it is valid and supported by the daemon.

Fixes #6918

Signed-off-by: mohithshuka <153504854+mohithshuka@users.noreply.github.com>

Author: mohithshuka

cli/command/volume/prune.go

@@ -57,7 +57,7 @@ func newPruneCommand(dockerCLI command.Cli) *cobra.Command {
 	flags.BoolVarP(&options.all, "all", "a", false, "Remove all unused volumes, not just anonymous ones")
 	flags.SetAnnotation("all", "version", []string{"1.42"})
 	flags.BoolVarP(&options.force, "force", "f", false, "Do not prompt for confirmation")
-	flags.Var(&options.filter, "filter", `Provide filter values (e.g. "label=<label>")`)
+	flags.Var(&options.filter, "filter", `Provide filter values (e.g. "label=<label>" or "label!=<label>")`)
 
 	return cmd
 }