Update README.md

README.md

@@ -404,7 +404,7 @@ db.count({}, function (err, count) {
   * The modifiers create the fields they need to modify if they don't exist, and you can apply them to subdocs. Available field modifiers are `$set` to change a field's value, `$unset` to delete a field and `$inc` to increment a field's value. To work on arrays, you have `$push`, `$pop`, `$addToSet`, `$pull`, and the special `$each`. See examples below for the syntax.
 * `options` is an object with two possible parameters
   * `multi` (defaults to `false`) which allows the modification of several documents if set to true
-  * `upsert` (defaults to `false`) if you want to insert a new document corresponding to the `update` rules if your `query` doesn't match anything
+  * `upsert` (defaults to `false`) if you want to insert a new document corresponding to the `update` rules if your `query` doesn't match anything. If your `update` is a simple object with no modifiers, it is the inserted document. In the other case, the `query` is stripped from all operator recursively, and the `update` is applied to it.
 * `callback` (optional) signature: `err`, `numReplaced`, `newDoc`
   * `numReplaced` is the number of documents replaced
   * `newDoc` is the created document if the upsert mode was chosen and a document was inserted