docs: improvements to source code documentation (#1236)

src/platform/macos/av_img_t.h

@@ -1,5 +1,4 @@
-#ifndef av_img_t_h
-#define av_img_t_h
+#pragma once
 
 #include "src/platform/common.h"
 
@@ -14,5 +13,3 @@ namespace platf {
     ~av_img_t();
   };
 }  // namespace platf
-
-#endif /* av_img_t_h */