[TTree][NFC] Improve TTree::Draw docs
[TTree][NFC] Improve TTree::Draw docs

Clarify behavior of `selection`s that return arrays.

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