doc(user/bkdatev):Add comment warning about macOS compatibility

- Note: Basically, the `bkdatev` command by itself should work with
  macOS 11.1. Additional arguments are passed along to `date` so the
  provided example (`bkdatev --date="2001-09-11T09:02:59-04"`) will fail
  due to BSD `date` not accepting ISO-8601 date strings like GNU
  Coreutils `date` does.