Clarify ContentMode javadocs (#10395)

* Don't refer to Label
* Explicitly mention XSS risk with HTML

shared/src/main/java/com/vaadin/shared/ui/ContentMode.java

@@ -16,24 +16,25 @@
 package com.vaadin.shared.ui;
 
 /**
- * Content modes defining how the client should interpret a Label's value.
+ * Defines how the client should interpret textual values.
  *
  * @since 8.0
  */
 public enum ContentMode {
     /**
-     * Content mode, where the label contains only plain text.
+     * Textual values are displayed as plain text.
      */
     TEXT,
 
     /**
-     * Content mode, where the label contains preformatted text. In this mode
-     * newlines are preserved when rendered on the screen.
+     * Textual values are displayed as preformatted text. In this mode newlines
+     * are preserved when rendered on the screen.
      */
     PREFORMATTED,
 
     /**
-     * Content mode, where the label contains HTML.
+     * Textual values are interpreted and displayed as HTML. Care should be
+     * taken when using this mode to avoid Cross-site Scripting (XSS) issues.
      */
     HTML