Clarifies db can be set up from structure.sql also

I added that *structure.sql* file can be used when *db:reset* is run.

*db:reset* tasks states *db:reset* loads database from *db/schema.rb*
or *db/structure.sql* depending on the configuration (although
*db/schema.rb* is the default), hence the change.

guides/source/active_record_migrations.md

@@ -789,7 +789,7 @@ The `rake db:reset` task will drop the database and set it up again. This is
 functionally equivalent to `rake db:drop db:setup`.
 
 NOTE: This is not the same as running all the migrations. It will only use the
-contents of the current `schema.rb` file. If a migration can't be rolled back,
+contents of the current `schema.rb` or `structure.rb` file. If a migration can't be rolled back,
 `rake db:reset` may not help you. To find out more about dumping the schema see
 [Schema Dumping and You](#schema-dumping-and-you) section.