docs(upsert): improve return description

lib/model.js

@@ -2427,7 +2427,7 @@ class Model {
    * @param  {boolean}      [options.benchmark=false]                     Pass query execution time in milliseconds as second argument to logging function (options.logging).
    * @param  {string}       [options.searchPath=DEFAULT]                  An optional parameter to specify the schema search_path (Postgres only)
    *
-   * @returns {Promise<Model,boolean | null>} returns record and whether row was created or updated as boolean. For Postgres/SQLite dialects boolean value is always null`.
+   * @returns {Promise<[Model, boolean | null]>} returns an array with two elements, the first being the new record and the second being `true` if it was just created or `false` if it already existed (except on Postgres and SQLite, which can't detect this and will always return `null` instead of a boolean).
    */
   static async upsert(values, options) {
     options = {