Add note re _NullPool

src/hb-open-type-private.hh

diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 5d90e5b..5f097f0 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -130,6 +130,7 @@
  */
 
 /* Global nul-content Null pool.  Enlarge as necessary. */
+/* TODO This really should be a extern HB_INTERNAL and defined somewhere... */
 static const void *_NullPool[64 / sizeof (void *)];
 
 /* Generic nul-content Null objects. */