[TTree][NFC][ROOT-4489] Document BuildIndex behaviour about preexisting indices
[TTree][NFC][ROOT-4489] Document BuildIndex behaviour about preexisting indices

i.e. the re-creation of the index without deleting the old one in order to prevent
accidental deletion of previously assigned indices.

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