Update definition of returned iterator of _it_from of RB tree

from the greatest element of 'rbtree' lower of equal than 'data'
to the lowest element of the tree 'rbtree' greater or equal than 'data'

It is more logical and natural:
if you have a tree of elements between [2, 4], and you want
to iterate all elements from 0 to 10, the old definition doesn't allow
you to iterate easily over this subset.

README.md

@@ -2818,7 +2818,7 @@ Set the iterator 'it' to no element of 'rbtree'.
 ##### void name\_it\_from(name\_it\_t it, const name\_t rbtree, const type data)
 
 Set the iterator 'it' to
-the greatest element of 'rbtree' lower of equal than 'data'
+the lowest element of the tree 'rbtree' greater or equal than 'data'
 or an iterator to no element is there is none.
 
 ##### bool name\_end\_p(const name\_it\_t it)