comment about ThreadLocal key or value types

Signed-off-by: Ceki Gulcu <ceki@qos.ch>

Author: ceki

slf4j-api/src/main/java/org/slf4j/helpers/ThreadLocalMapOfStacks.java

@@ -15,6 +15,9 @@
  */
 public class ThreadLocalMapOfStacks {
 
+    // BEWARE: Keys or values placed in a ThreadLocal should not be of a type/class
+    // not included in the JDK. See also https://jira.qos.ch/browse/LOGBACK-450
+
     final ThreadLocal<Map<String, Deque<String>>> tlMapOfStacks = new ThreadLocal<>();
 
     public void pushByKey(String key, String value) {