[subset] add note about memory management with preprocessed faces.

src/hb-subset-input.cc

@@ -456,6 +456,11 @@ hb_subset_input_pin_axis_location (hb_subset_input_t  *input,
  * subsetter. Future subsetting operations can then use the precomputed data
  * to speed up the subsetting operation. Returns a new hb_face_t*.
  *
+ * Note: the preprocessed face may contain sub-blobs that reference the memory
+ * backing the source hb_face_t*. Therefore in the case that this memory is not
+ * owned by the source face you will need to ensure that memory lives
+ * as long as the returned hb_face_t*.
+ *
  * Since: REPLACEME
  **/