[Tree][Docs] Document TTree::Branch().
[Tree][Docs] Document TTree::Branch().

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