add one comment when destroy the buffer.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@8322 6f19259b-4bc3-4df7-8a09-765794883524

UnixPkg/Sec/UgaX11.c

@@ -170,6 +170,7 @@ UgaSize(EFI_UNIX_UGA_IO_PROTOCOL *UgaIo, UINT32 Width, UINT32 Height)
   /* Destroy current buffer if created.  */
   if (drv->image != NULL)
     {
+      /* Before destroy buffer, need to make sure the buffer available for access. */
       XDestroyImage(drv->image);
 
       if (drv->use_shm)