Doxygen is a software documentation generator system for multiple programming languages. It can generate documentation in different formats (such as HTML, Latex, RTF, PostScript). The documentation tool is useful for the developers, because the documentation is inlined in the source code, which makes it easy to keep in sync with the new revisions of the project. It walks up the source files and even there is no documentation provided it can generate description in terms of UML diagrams - particularly useful when dealing with large codebases.
Cling has online doxygen documentation that could be found here
If you want to be able to browse offline the doxygen documentation you can build your own with:
cd CLING_BUILD_DIR/docs/ make regendoc