Renamed IExecutor -> CommandExecutor and fixed SERVER_COMMAND doc

By: stevenh <steven.hartland@multiplay.co.uk>

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

@@ -10,10 +10,16 @@ public interface CommandSender {
     public void sendMessage(String message);
 
     /**
-     * Checks if this player is currently op
+     * Checks if this sender is currently op
      *
      * @return true if they are
      */
     public boolean isOp();
 
+    /**
+     * Checks if this sender is a player
+     *
+     * @return true if they are
+     */
+    public boolean isPlayer();
 }