docs: documentation for "-c" connection option (typeorm#6080)

Add documentantion for this typeorm#1333

docs/migrations.md

@@ -4,6 +4,7 @@
 * [Creating a new migration](#creating-a-new-migration)
 * [Running and reverting migrations](#running-and-reverting-migrations)
 * [Generating migrations](#generating-migrations)
+* [Connection option](#connection-option)
 * [Using migration API to write migrations](#using-migration-api-to-write-migrations)
 
 ## How migrations work
@@ -196,6 +197,12 @@ export class PostRefactoringTIMESTAMP implements MigrationInterface {
 See, you don't need to write the queries on your own.
 The rule of thumb for generating migrations is that you generate them after "each" change you made to your models.
 
+## Connection option
+If you need to run/revert your migrations for another connection rather than the default, use the `-c` (alias for `--connection`) and pass the config name as an argument
+```
+typeorm -c <your-config-name> migration:{run|revert}
+```
+
 ## Using migration API to write migrations
 
 In order to use an API to change a database schema you can use `QueryRunner`.