You are asking the wrong question. The kind of documents you are asking for typically does not exist for open source projects. (They would exist in a project that was using object-oriented, model driven development, where the whole project was specified up front, then implemented in a finite period of time.) You may need to adjust your thinking to get into the OS mindset (the wrintings of Eric Raymond may help: http://www.catb.org/~esr/
The documentation that does exist for Moodle is on docs.moodle.org. You will probably be most interested in the Developer section. Bear in mind that this site is a wiki, which means that everyone can contribute. If you find that somethings you are intersted in is not explained properly, and you puzzle it out yourself, you can help the community by adding to the documentation.