8000665: fix "internal API" comments on javadoc files

Reviewed-by: darcy

langtools/src/share/classes/com/sun/tools/javadoc/JavadocMemberEnter.java

@@ -37,6 +37,12 @@ import com.sun.tools.javac.util.Position;
 /**
  *  Javadoc's own memberEnter phase does a few things above and beyond that
  *  done by javac.
+ *
+ *  <p><b>This is NOT part of any supported API.
+ *  If you write code that depends on this, you do so at your own risk.
+ *  This code and its internal interfaces are subject to change or
+ *  deletion without notice.</b>
+ *
  *  @author Neal Gafter
  */
 public class JavadocMemberEnter extends MemberEnter {