Add Javadoc to PartiallyOrderedSet#getNonChildren and getNonParents to clarify their behavior

Than naming of the methods in conjunction with the following Javadoc comment

 * <p>In addition to the usual set methods, there are methods to determine the
 * immediate parents and children of an element in the set, and method to find
 * all elements which have no parents or no children (i.e. "root" and "leaf"
 * elements).

led me to the wrong conclusion that they do the following:

• getNonChildren return all elements which have no children (leafs)
• getNonParents return all elements which have no parents (roots)

while they do the opposite.

The name is subject to different interpretations so to avoid similar confusion in the future we should add some explicit documentation to the methods.