[docs] Fix comment syntax

To lower the number of gtk-doc warnings.

src/hb-ot-layout.cc

diff --git a/src/hb-ot-layout.cc b/src/hb-ot-layout.cc
index 3049fa1..9fc88f6 100644
--- a/src/hb-ot-layout.cc
+++ b/src/hb-ot-layout.cc
@@ -458,6 +458,8 @@
 }
 
 /**
+ * hb_ot_layout_feature_get_lookups:
+ *
  * Since: 0.9.7
  **/
 unsigned int
@@ -475,6 +477,8 @@
 }
 
 /**
+ * hb_ot_layout_table_get_lookup_count:
+ *
  * Since: 0.9.22
  **/
 unsigned int
@@ -635,6 +639,8 @@
 }
 
 /**
+ * hb_ot_layout_collect_lookups:
+ *
  * Since: 0.9.8
  **/
 void
@@ -679,6 +685,8 @@
 }
 
 /**
+ * hb_ot_layout_lookup_collect_glyphs:
+ *
  * Since: 0.9.7
  **/
 void
@@ -727,6 +735,8 @@
 }
 
 /**
+ * hb_ot_layout_lookup_would_substitute:
+ *
  * Since: 0.9.7
  **/
 hb_bool_t
@@ -768,6 +778,8 @@
 }
 
 /**
+ * hb_ot_layout_lookup_substitute_closure:
+ *
  * Since: 0.9.7
  **/
 void
@@ -805,6 +817,8 @@
 }
 
 /**
+ * hb_ot_layout_get_size_params:
+ *
  * Since: 0.9.10
  **/
 hb_bool_t