Add note on size units in file_operations.h

Change-Id: I59adb55cf6d8b494d3e0c1ccc4d6792c4dbc4980
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/events/logging/file_operations.h b/aos/events/logging/file_operations.h
index 538ae60..0c2133d 100644
--- a/aos/events/logging/file_operations.h
+++ b/aos/events/logging/file_operations.h
@@ -16,7 +16,7 @@
  public:
   struct File {
     std::string name;
-    size_t size;
+    size_t size;  // bytes.
   };
 
   virtual ~FileOperations() = default;