docs: improve docs and add explanation for flux (#2928)

flux/src/main/java/com/iluwatar/flux/action/ContentAction.java

@@ -24,19 +24,18 @@
  */
 package com.iluwatar.flux.action;
 
+import lombok.Getter;
+
 /**
  * ContentAction is a concrete action.
  */
 public class ContentAction extends Action {
 
+  @Getter
   private final Content content;
 
   public ContentAction(Content content) {
     super(ActionType.CONTENT_CHANGED);
     this.content = content;
   }
-
-  public Content getContent() {
-    return content;
-  }
 }