docs(addIndex): concurrently option (#10409)

lib/dialects/abstract/query-generator.js

@@ -476,6 +476,7 @@ class QueryGenerator {
                   - length: An integer. Optional
                   - order: 'ASC' or 'DESC'. Optional
         - parser
+        - concurrently: Pass CONCURRENT so other operations run while the index is created
       - rawTablename, the name of the table, without schema. Used to create the name of the index
    @private
   */

lib/query-interface.js

@@ -619,6 +619,7 @@ class QueryInterface {
    * @param {Array}   [attributes]     Use options.fields instead, List of attributes to add index on
    * @param {Object}  options          indexes options
    * @param {Array}   options.fields   List of attributes to add index on
+   * @param {boolean} [options.concurrently] Pass CONCURRENT so other operations run while the index is created
    * @param {boolean} [options.unique] Create a unique index
    * @param {string}  [options.using]  Useful for GIN indexes
    * @param {string}  [options.type]   Type of index, available options are UNIQUE|FULLTEXT|SPATIAL