docs(virtualtype): document count option

lib/virtualtype.js

@@ -16,6 +16,7 @@
  * @param {string|function} [options.foreignField] the foreign field to populate on if this is a populated virtual.
  * @param {boolean} [options.justOne=false] by default, a populated virtual is an array. If you set `justOne`, the populated virtual will be a single doc or `null`.
  * @param {boolean} [options.getters=false] if you set this to `true`, Mongoose will call any custom getters you defined on this virtual
+ * @param {boolean} [options.count=false] if you set this to `true`, `populate()` will set this virtual to the number of populated documents, as opposed to the documents themselves, using [`Query#countDocuments()`](./api.html#query_Query-countDocuments)
  * @api public
  */