[RF][Docs] Add formula to RooDSTD0 docs.
[RF][Docs] Add formula to RooDSTD0 docs.

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