[meta] Add metadata tags documentation from ot spec

src/hb-ot-meta.h

diff --git a/src/hb-ot-meta.h b/src/hb-ot-meta.h
index 7afad84..83fcc04 100644
--- a/src/hb-ot-meta.h
+++ b/src/hb-ot-meta.h
@@ -35,8 +35,14 @@
 
 /**
  * hb_ot_meta_tag_t:
+ * @HB_OT_META_TAG_DESIGN_LANGUAGES: Design languages. Text, using only
+ * Basic Latin (ASCII) characters. Indicates languages and/or scripts
+ * for the user audiences that the font was primarily designed for.
+ * @HB_OT_META_TAG_SUPPORTED_LANGUAGES: Supported languages. Text, using
+ * only Basic Latin (ASCII) characters. Indicates languages and/or scripts
+ * that the font is declared to be capable of supporting.
  *
- * From https://docs.microsoft.com/en-us/typography/opentype/spec/meta
+ * Known metadata tags from https://docs.microsoft.com/en-us/typography/opentype/spec/meta
  *
  * Since: REPLACEME
  **/