Update document of _it_from, _it_until_p, _it_while_p:
Update document of _it_from, _it_until_p, _it_while_p:

* _it_from: if there is no element that matches the condition,
it returns a null iterator, which is more coherent.
* _it_until_p: clarify condition if the iterator is null.
* _it_while_p: clarify condition if the iterator is null.

P-p-H-d/mlib,P-p-H-d/mlib