[RF][Docs] Fix documentation of RooDataHist.
[RF][Docs] Fix documentation of RooDataHist.

[ROOT-10784] The set() functions of RDH all had copy-pasted documentation
that was completely wrong.

olifre/root,root-mirror/root,karies/root,karies/root,karies/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root