docs(model): clean up some docs re: aggregate()

Author: vkarpov15

lib/model.js

@@ -3885,9 +3885,8 @@ Model.mapReduce = function mapReduce(o, callback) {
  *
  * ####NOTE:
  *
- * - Arguments are not cast to the model's schema because `$project` operators allow redefining the "shape" of the documents at any stage of the pipeline, which may leave documents in an incompatible format.
+ * - Mongoose does **not** cast aggregation pipelines to the model's schema because `$project` and `$group` operators allow redefining the "shape" of the documents at any stage of the pipeline, which may leave documents in an incompatible format. You can use the [mongoose-cast-aggregation plugin](https://github.com/AbdelrahmanHafez/mongoose-cast-aggregation) to enable minimal casting for aggregation pipelines.
  * - The documents returned are plain javascript objects, not mongoose documents (since any shape of document can be returned).
- * - Requires MongoDB >= 2.1
  *
  * @see Aggregate #aggregate_Aggregate
  * @see MongoDB http://docs.mongodb.org/manual/applications/aggregation/