Cleanup: use 'pragma once'

Add explanations for cases the header-guard defines are still used.

source/blender/blenlib/BLI_memory_utils.h

@@ -14,6 +14,7 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  */
 
+/* Use a define instead of `#pragma once` because of `BLI_utildefines.h` */
 #ifndef __BLI_MEMORY_UTILS_H__
 #define __BLI_MEMORY_UTILS_H__