document both cache class loader the ApcClassLoader and the XcacheClassLoader
documentation for the DebugClassLoader
documentation for the MapClassLoader class
add an introduction and documentation for the ClassLoader class
add versionadded directives for new classes
fixes thanks to @WouterJ
fix markup of links
fix link to Class Loader docs
Merge pull request #2913 from xabbuh/issue-2834

documentation for the Class Loader component