[DF][NFC] Bring back doxygen docs for ROOT::RDF::ColumnNames_t (#8855)
[DF][NFC] Bring back doxygen docs for ROOT::RDF::ColumnNames_t (#8855)

For unclear reasons the `\cond` directive put _after_ the declaration
of the `ColumnNames_t` alias inhibited docs for it being created.
olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root