[DF] Give more meaningful user-visible names to Fill actions
[DF] Give more meaningful user-visible names to Fill actions

In particular, point out when we are doing something "special", i.e.
filling histograms with unkown axes. This requires extra care and
extra memory usage in multi-thread runs.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root