docs: more context on vol rm

See coop-cloud/organising#265.

cli/app/volume.go

@@ -41,8 +41,19 @@ var appVolumeListCommand = &cli.Command{
 }
 
 var appVolumeRemoveCommand = &cli.Command{
-	Name:    "remove",
-	Usage:   "Remove volume(s) associated with an app",
+	Name:  "remove",
+	Usage: "Remove volume(s) associated with an app",
+	Description: `
+This command supports removing volumes associated with an app. The app in
+question must be undeployed before you try to remove volumes. See "abra app
+undeploy <app>" for more.
+
+The command is interactive and will show a multiple select input which allows
+you to make a seclection. Use the "?" key to see more help on navigating this
+interface.
+
+Passing "--force" will select all volumes for removal. Be careful.
+`,
 	Aliases: []string{"rm"},
 	Flags: []cli.Flag{
 		internal.ForceFlag,