minor doc improvement for tombstone fd code

Change-Id: I8037d9dd5a6c0e86e853d001ccedc472ce61d107
diff --git a/debuggerd/tombstoned/tombstoned.cpp b/debuggerd/tombstoned/tombstoned.cpp
index cf7904f..e79035f 100644
--- a/debuggerd/tombstoned/tombstoned.cpp
+++ b/debuggerd/tombstoned/tombstoned.cpp
@@ -417,6 +417,7 @@
     return false;
   }
 
+  // This fd is created inside of dirfd in CrashQueue::create_temporary_file.
   std::string fd_path = StringPrintf("/proc/self/fd/%d", fd.get());
   rc = linkat(AT_FDCWD, fd_path.c_str(), dirfd.get(), path.c_str(), AT_SYMLINK_FOLLOW);
   if (rc != 0) {