More doc on the _id collision probability

lib/customUtils.js

@@ -22,6 +22,8 @@ function ensureDirectoryExists (dir, cb) {
  * There is a very small probability (less than 1/1,000,000) for the length to be less than len
  * (il the base64 conversion yields too many pluses and slashes) but
  * that's not an issue here
+ * The probability of a collision is extremely small (need 3*10^12 documents to have one chance in a million of a collision)
+ * See http://en.wikipedia.org/wiki/Birthday_problem
  */
 function uid (len) {
   return crypto.randomBytes(Math.ceil(Math.max(8, len * 2)))