[docs] Hide private enum values

"/*< skip >*/" does not seem to hide them.
diff --git a/src/hb-ot-layout.h b/src/hb-ot-layout.h
index 545d5f7..54dd780 100644
--- a/src/hb-ot-layout.h
+++ b/src/hb-ot-layout.h
@@ -446,6 +446,7 @@
   HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_TOP_OR_RIGHT	= HB_TAG ('i','d','t','p'),
   HB_OT_LAYOUT_BASELINE_TAG_MATH			= HB_TAG ('m','a','t','h'),
 
+  /*< private >*/
   _HB_OT_LAYOUT_BASELINE_TAG_MAX_VALUE = HB_TAG_MAX_SIGNED /*< skip >*/
 } hb_ot_layout_baseline_tag_t;