Reference documentation

There is no documentation for site. I mean there is some, but this is not enough. For example there is no reference what elements can be included in site.xml. What are tags? No clue. It is not possible to use it as it is now. I do understand I can find out what they do by trying, but at least I need to know what they are. I tried to search on internet for any documentation, but no success. It is very frustrating. I think many users give up using it, just because they can not find what it can do and how to use it.
Please provide at least simple list of the elements that are available in site.xml.