Earlier version of Docman included a plugin called Docman Notify, but according to the developers website,
- Notify plugin doesn't work in 2.0. We are working on a new Notify plugin. This is in the works.
Source: Joomlatools forum
Although you don't want to use commercial extensions, Content Update Notifications seems to be your best option.
Another (free) alternative is NotifyArticleSubmit. I haven't used it myself, and I don't now if it works with Docman, but if it doesn't you might be able to modify it to fit your needs.
As others pointed out, you could also build your own plugin, using any of the existing events as needed:
- onContentBeforeDelete : This is an event that is called right before the content is deleted.
- onContentAfterDelete : This is an event that is called right after the content is deleted.
- onContentBeforeDisplay : This is a request for information that should be placed immediately before the generated content.
- onContentAfterDisplay: This is a request for information that should be placed immediately after the generated content.
- onContentBeforeSave : This is an event that is called right before the content is saved into the database.
- onContentAfterSave : This is an event that is called after the content is saved into the database.
- onContentAfterTitle : This is a request for information that should be placed between the content title and the content body.
- onContentChangeState : This is an event that is called when the contents state is changed.
- onContentPrepare : This is the first stage in preparing content for output and is the most common point for content orientated plugins to do their work.
More info on plugin development can be found here.