I remember how many nights I spend to investigate the source code of Zabbix. These were the times where I learned most about Zabbix. This was real fun but consumed a significant portion of my time.
Completing the description of major functions as well as adding some more comments to certain function calls resp. key areas in a function could ease a lot to get into it and to finally understand.
PS: My personal preference would be Sphinx resp. use of reStructuredText