[gobject] Fix a few warnings

Part of https://github.com/behdad/harfbuzz/issues/277
diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 406db9c..4552d15 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -52,7 +52,7 @@
  *
  * Checks the equality of two #hb_segment_properties_t's.
  *
- * Return value: (transfer full):
+ * Return value:
  * %true if all properties of @a equal those of @b, false otherwise.
  *
  * Since: 0.9.7