TreeMap examples fixes

1. Removed obsolete comment regarding recursive/iteration implementations of tree_find_with/tree_find_mut_with
2. Replaced easy breakable find_with example with simpler one (which only removes redundant allocation during search)
auto merge of #15749 : vhbit/rust/treemap-doc-fixes, r=alexcrichton

1. Removed obsolete comment regarding recursive/iteration implementations of tree_find_with/tree_find_mut_with
2. Replaced easy breakable find_with example with simpler one (which only removes redundant allocation during search)