Documented possible exception being thrown in ClassLoader destructor.

[remod]

The class member class_loader::MultiLibraryClassLoader lowlevel_class_loader_ can throw an exception when being destroyed (which actually happened to me). This can be hard to fix, especially if one defines the ClassLoader instance to be static. In this case the instance is destroyed when the executable exits where you can't catch exceptions.

Documenting this behavior should help others to be careful. Also it is consistent with the documentation of the other methods.