Using phpxref tool, XOOPS 2.2 developer doc is published at http://www.xoops.info . It will be a reference for module developer, beginner and experinced users alike. Besides file level code doc, it has context specific code and search for classes, variable, functions and constants.
Summary of XOOPS 2.2 is follow.

Summary statistics:
Files Scanned: 699 containing 92658 lines
2854 variable names in 7736 definitions and 49852 references.
1735 function/method names in 1983 definitions and 24114 references.
1939 constant names in 2010 definitions and 6381 references.
230 class names in 230 definitions and 1758 references.

All criticism welcome.

thanks