Document whether to set `safe: true` or `false`

[jonathanong]

MongoDB 1.1.10 requires you to explicitly set safe in the db_options. You can set it to false. If you don't set it, this annoying message pops up: https://github.com/mongodb/node-mongodb-native/blob/master/lib/mongodb/db.js#L111

Right now I'm setting it to safe: false, but in the future I believe safe: true by default, so you would want to update the code accordingly.