Update ServerSocket.isBound spec to reflect implementation after close

Summary

Documentation clarification in java.net.ServerSocket. No behavior change.

Problem

In a previous change (6505016) most of the other socket types had the same clarification added but this was overlooked. ServerSocket::isBound returns true after a socket was bound but is now closed. The docs need to state this explicitly

Solution

Doc change as shown below

Specification

Add text shown to ServerSocket::isBound apidoc:

diff -r 40006c0ada91 src/java.base/share/classes/java/net/ServerSocket.java
--- a/src/java.base/share/classes/java/net/ServerSocket.java    Wed Aug 21 00:08:35 2019 +0200
+++ b/src/java.base/share/classes/java/net/ServerSocket.java    Wed Aug 21 16:40:44 2019 +0100
@@ -710,6 +710,10 @@

     /**
      * Returns the binding state of the ServerSocket.
+     * <p>
+     * If the socket was bound prior to being {@linkplain #close closed},
+     * then this method will continue to return {@code true}
+     * after the socket is closed.
      *
      * @return true if the ServerSocket successfully bound to an address
      * @since 1.4