docs: improvements to source code documentation (#1236)

src/thread_pool.h

@@ -1,14 +1,12 @@
-#ifndef KITTY_THREAD_POOL_H
-#define KITTY_THREAD_POOL_H
+#pragma once
 
 #include "task_pool.h"
 #include <thread>
 
 namespace thread_pool_util {
-  /*
- * Allow threads to execute unhindered
- * while keeping full control over the threads.
- */
+  /**
+   * Allow threads to execute unhindered while keeping full control over the threads.
+   */
   class ThreadPool: public task_pool_util::TaskPool {
   public:
     typedef TaskPool::__task __task;
@@ -127,4 +125,3 @@ namespace thread_pool_util {
     }
   };
 }  // namespace thread_pool_util
-#endif