[#2323] Make it clear a Channel must be closed to release all resources

Motivation:
Ensure the user know the Channel must be closed to release resources like filehandles.

Modifications:
Add some extra javadoc.

Result:
More clear documentation
[#2326] Add constructor to NioServerSocketChannel which accepts a ServerSocketChannel

Motivation:
Allow the user to create a NioServerSocketChannel from an existing ServerSocketChannel.

Modifications:
Add an extra constructor

Result:
Now the user is be able to create a NioServerSocketChannel from an existing ServerSocketChannel, like he can do with all the other Nio*Channel implemntations.