Apache Solr support for custom entities
If you want to get an easy to use search, you should consider using Search API where you can use a Solr backend and index entities, fields and even relations to other entities.

The apache solr module is more an API to Solr then a ready made search engine (though the module does offer some various search addons), which means you can index whatever you like, but you might need to write some code for custom entities.