docs: note the preserved ENV file can be deleted

The preserved copy holds everything the legacy file was not allowed to import, which includes keys that had been `config:unset` on purpose, so a revoked secret can outlive its revocation in a file `dokku config:*` no longer reads. Nothing said the copy was the operator's to remove once reviewed.

tests/unit/config.bats

@@ -644,6 +644,7 @@ teardown() {
   assert_success
   assert_output_contains "the current config takes precedence for KEEP RESURRECTED"
   assert_output_contains "$DOKKU_ROOT/$TEST_APP/ENV.migrated"
+  assert_output_contains "then delete it"
 
   run /bin/bash -c "dokku config:get $TEST_APP KEEP"
   echo "output: $output"