[docs] Complete hb-unicode docs a bit
diff --git a/src/hb-unicode.h b/src/hb-unicode.h
index 3cfeab1..23e7b5f 100644
--- a/src/hb-unicode.h
+++ b/src/hb-unicode.h
@@ -41,7 +41,9 @@
 
 
 /**
- * HB_UNICODE_MAX
+ * HB_UNICODE_MAX:
+ *
+ * Maximum valid Unicode code point.
  *
  * Since: 1.9.0
  **/