diff --git a/src/hb-face.cc b/src/hb-face.cc
index 5395bf85f..97582face 100644
--- a/src/hb-face.cc
+++ b/src/hb-face.cc
@@ -471,6 +471,8 @@ hb_face_get_index (const hb_face_t *face)
  *
  * Sets the units-per-em (upem) for a face object to the specified value.
  *
+ * This API is used in rare circumstances.
+ *
  * Since: 0.9.2
  **/
 void
@@ -509,6 +511,8 @@ hb_face_get_upem (const hb_face_t *face)
  *
  * Sets the glyph count for a face object to the specified value.
  *
+ * This API is used in rare circumstances.
+ *
  * Since: 0.9.7
  **/
 void