1. java.net.Socket class level API documentation says that SO_LINGER:
"Linger on close if data is present (when configured in blocking mode only)"
The last part of the sentence (when configured in blocking mode only) is not really pertinent here - as sockets are typically configured in blocking mode by default. A more complete information can be obtained by reading the API documentation of StandardSocketOptions.SO_LINGER anyway.

2. MulticastSocket now only has a handful of methods that are not deprecated. It could be a good idea to add some links to the corresponding methods in DatagramChannel and point out that an alternative way of receiving multicast packets is to use the MulticastChannel functionality of DatagramChannel instead. DatagramChannel supports IGMPv3 while MulticastSocket does not (see JDK-4403551).