For systems we use and develop or modify ourselves, we have this template for needed documentation. Whether each of the following is a separate page, or not, is specific to the size of the documentation.
Whether all categories are included depends upon the software. E.g. if it's not a CGI, then there will be no CGI parameters to document. And if it doesn't use a database, then there will be no schema to document.
Examples of this are:
This is (as of 2015-09) a work in progress.