Merge pull request #16593 from aditya-kapoor/add-doc-for-step

[ci skip] add doc for STEP in migration

activerecord/lib/active_record/migration.rb

@@ -181,9 +181,12 @@ def initialize
   #
   # To roll the database back to a previous migration version, use
   # <tt>rake db:migrate VERSION=X</tt> where <tt>X</tt> is the version to which
-  # you wish to downgrade. If any of the migrations throw an
-  # <tt>ActiveRecord::IrreversibleMigration</tt> exception, that step will fail and you'll
-  # have some manual work to do.
+  # you wish to downgrade. Alternatively, you can also use the STEP option if you
+  # wish to rollback last few migrations. <tt>rake db:migrate STEP=2</tt> will rollback
+  # the latest two migrations.
+  # 
+  # If any of the migrations throw an <tt>ActiveRecord::IrreversibleMigration</tt> exception, 
+  # that step will fail and you'll have some manual work to do.
   #
   # == Database support
   #