Added warning to Labeler for developers who want to implement it.

core/src/main/java/ch/dissem/bitmessage/ports/Labeler.java

@@ -20,6 +20,8 @@ import ch.dissem.bitmessage.entity.Plaintext;
 
 /**
  * Defines and sets labels. Note that it should also update the status field of a message.
+ * Generally it's highly advised to override the {@link DefaultLabeler} whenever possible,
+ * instead of directly implementing the interface.
  * <p>
  * As the labeler gets called whenever the state of a message changes, it can also be used
  * as a listener.
@@ -35,6 +37,10 @@ public interface Labeler {
 
     void markAsDraft(Plaintext msg);
 
+    /**
+     * It is paramount that this methods marks the {@link Plaintext} object with status
+     * {@link Plaintext.Status#PUBKEY_REQUESTED} (see {@link DefaultLabeler})
+     */
     void markAsSending(Plaintext msg);
 
     void markAsSent(Plaintext msg);