8268087: Update documentation of the JPasswordField

Reviewed-by: trebari, azvegint, prr

src/java.desktop/share/classes/javax/swing/JPasswordField.java

@@ -60,6 +60,14 @@ import javax.swing.text.Segment;
  * If an application needs the input methods support, please use the
  * inherited method, <code>enableInputMethods(true)</code>.
  * <p>
+ * <strong>Warning:</strong> The {@code JPasswordField} will not show the
+ * original characters that were typed, instead displaying alternative text or
+ * graphics. However this doesn't prevent the password from appearing in the
+ * system memory. For handling confidential information such as the password
+ * text, refer to the relevant section at
+ * <a href="https://www.oracle.com/java/technologies/javase/seccodeguide.html">
+ * Secure Coding Guidelines</a>.
+ * <p>
  * <strong>Warning:</strong> Swing is not thread safe. For more
  * information see <a
  * href="package-summary.html#threading">Swing's Threading