In a language pack or with the Linguist extension, you can edit the key core.lib.error.rate_limit_exceeded_message
To add more details to the error message would require writing a Flarum extension though, and I don't know if you can use the rate limit API at all because it might not support additional attributes. You might need to write your own validation error to be thrown from the Saving event, and leave the default rate limit disabled.
