updated doc [skip ci]

docs/guide/db-migrations.md

@@ -293,7 +293,7 @@ line:
   migration history information. It defaults to `migration`. The table
   structure is `version varchar(255) primary key, apply_time integer`.
 
-* `connectionID`: string, specifies the ID of the database application component.
+* `db`: string, specifies the ID of the database application component.
   Defaults to 'db'.
 
 * `templateFile`: string, specifies the path of the file to be served as the code

framework/db/Migration.php

@@ -40,7 +40,10 @@ class Migration extends Component implements MigrationInterface
 {
     /**
      * @var Connection|string the DB connection object or the application component ID of the DB connection
-     * that this migration should work with.
+     * that this migration should work with. Note that when a Migration object is created by
+     * the `migrate` command, this property will be overwritten by the command. If you do not want to
+     * use the DB connection provided by the command, you may override the [[init()]] method
+     * and set this property after calling `parent::init()`.
      */
     public $db = 'db';