docs: Improve Javadoc for HostAndPortMapper (#4112)

Author: JuneYub

src/main/java/redis/clients/jedis/HostAndPortMapper.java

@@ -1,6 +1,17 @@
 package redis.clients.jedis;
 
+/**
+ * An interface for mapping Redis node addresses.
+ * <p>
+ * It is used to translate an advertised server address to one that is reachable by the client,
+ * especially in network topologies involving NAT or containerization.
+ */
+@FunctionalInterface
 public interface HostAndPortMapper {
 
+  /**
+   * @param hap The original address from the server.
+   * @return The translated, reachable address.
+   */
   HostAndPort getHostAndPort(HostAndPort hap);
 }