[PPL-devel] Exploiting a feature of doxygen.

Reading the doxygen manual, I discovered the special command \relates that can be usefully exploited for placing the documentation of non-friend but related functions inside the section pertaining to a particular class. To me, using this command is better than having the explicit grouping of these functions inside the namespace documentation section.

The only drawback is that this way we can only document functions (e.g., it seems not possible to apply the \relates command to an enumerated type).