guides, include note about modifiers when using the CLI.

[ci skip]

This was pointed out on
https://github.com/rails/rails/issues/15583#issuecomment-239212831

The comment was lost in the changes made by
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guides/source/active_record_migrations.md

@@ -467,6 +467,8 @@ the first time (i.e. on the date the migration is applied).
 Some adapters may support additional options; see the adapter specific API docs
 for further information.
 
+NOTE: `null` and `default` cannot be specified via command line.
+
 ### Foreign Keys
 
 While it's not required you might want to add foreign key constraints to