Added new comment methods to mirgrations docs

docs/guide/db-migrations.md

@@ -685,6 +685,10 @@ Below is the list of all these database accessing methods:
 * [[yii\db\Migration::dropForeignKey()|dropForeignKey()]]: removing a foreign key
 * [[yii\db\Migration::createIndex()|createIndex()]]: creating an index
 * [[yii\db\Migration::dropIndex()|dropIndex()]]: removing an index
+* [[yii\db\Migration::addCommentOnColumn()|addCommentOnColumn()]: adding comment to column
+* [[yii\db\Migration::dropCommentFromColumn()|dropCommentFromColumn()]: dropping comment from column
+* [[yii\db\Migration::addCommentOnTable()|addCommentOnTable()]: adding comment to table
+* [[yii\db\Migration::dropCommentFromTable()|dropCommentFromTable()]: dropping comment from table
 
 > Info: [[yii\db\Migration]] does not provide a database query method. This is because you normally do not need
   to display extra message about retrieving data from a database. It is also because you can use the powerful