Clarify what "return false" in onCommand does.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/command/CommandExecutor.java

@@ -6,7 +6,10 @@ package org.bukkit.command;
 public interface CommandExecutor {
 
     /**
-     * Executes the given command, returning its success
+     * Executes the given command, returning its success.
+     * <br>
+     * If false is returned, then the "usage" plugin.yml entry for this command
+     * (if defined) will be sent to the player.
      *
      * @param sender Source of the command
      * @param command Command which was executed