TreeMap examples fixes

[vhbit]

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)

[olivren]

Changing the example is a good thing. However, there really should be a warning note to explain the restrictions that exist on this method. Here is my proposition:

Beware that this method is not a general purpose find implementation. It exists only as a mean to optimize a call to the regular find method, by allowing the caller to avoid performing a type conversion if it is not necessary. This method cannot be used to provide an alternative ordering logic (for example, searching a String key case-insensitively).

I'm not a native english speaker, so anyone feel free to correct it.

[Gankra]

Thanks for fixing this! :D

[vhbit]

Added a warning about natural ordering, squashed commits