Even if you aren't in the correct forum the answer remains the same. You were detained by police. You did not disclose this. You really need to send an updated form and hope they accept it. If they do not accept it then it is possible you will be denied PR based on misrepresentation because they WILL know about it. Never ever base your answers on what your police check says. CIC has a much more in depth way of looking at and finding things.