container: improve doc comments

Putting them on the builder methods is more useful.

Signed-off-by: Ophestra <cat@gensokyo.uk>

helper/container_test.go

@@ -14,7 +14,7 @@ func TestContainer(t *testing.T) {
 	t.Run("start empty container", func(t *testing.T) {
 		h := helper.New(t.Context(), container.Nonexistent, argsWt, false, argF, nil, nil)
 
-		wantErr := "sandbox: starting an empty container"
+		wantErr := "container: starting an empty container"
 		if err := h.Start(); err == nil || err.Error() != wantErr {
 			t.Errorf("Start: error = %v, wantErr %q",
 				err, wantErr)