ewrite the last section of the overwiew fo SortedMap documentation and a link fo NavigableMap

[Remi Forax]

Hello,
I was discussing with a student, she was trying to use the SortedMap interface but had trouble with tailMap() no being a strictly higher view from a key.
The solution was to use NavigableMap instead of SortedMap because it provides more methods to get lower/higher subMap and entries from keys.

I think we can improve de documentation of SortedMap (the last section (Note:) of the overview) by rewriting it saying that NavigableMap provides a richer API
instead of the current documentation that propose to add 1 or '\0' to the subMap() which is a ugly hack.

regards,
Rémi