doc: more javadoc

Author: next-guillermopriego

http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/ProxyConfiguration.java

@@ -67,10 +67,16 @@ public int port() {
         return port;
     }
 
+    /**
+     * @return The proxy username.
+     */
     public String username() {
         return username;
     }
 
+    /**
+     * @return The proxy password.
+     */
     public String password() {
         return password;
     }