[base] Add documentation for baseline tags from ot spec

src/hb-ot-layout.h

@@ -408,8 +408,25 @@ hb_ot_layout_feature_get_characters (hb_face_t      *face,
 
 /**
  * hb_ot_layout_baseline_tag_t:
+ * @HB_OT_LAYOUT_BASELINE_TAG_ROMAN: The baseline used by alphabetic scripts such as Latin, Cyrillic and Greek.
+ * In vertical writing mode, the alphabetic baseline for characters rotated 90 degrees clockwise.
+ * (This would not apply to alphabetic characters that remain upright in vertical writing mode, since these
+ * characters are not rotated.)
+ * @HB_OT_LAYOUT_BASELINE_TAG_HANGING: The hanging baseline. In horizontal direction, this is the horizontal
+ * line from which syllables seem, to hang in Tibetan and other similar scripts. In vertical writing mode,
+ * for Tibetan (or some other similar script) characters rotated 90 degrees clockwise.
+ * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_BOTTOM_OR_LEFT: Ideographic character face bottom or left edge,
+ * if the direction is horizontal or vertical, respectively.
+ * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_TOP_OR_RIGHT: Ideographic character face top or right edge,
+ * if the direction is horizontal or vertical, respectively.
+ * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_BOTTOM_OR_LEFT: Ideographic em-box bottom or left edge,
+ * if the direction is horizontal or vertical, respectively.
+ * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_TOP_OR_RIGHT: Ideographic em-box top or right edge baseline,
+ * if the direction is horizontal or vertical, respectively.
+ * @HB_OT_LAYOUT_BASELINE_TAG_MATH: The baseline about which mathematical characters are centered.
+ * In vertical writing mode when mathematical characters rotated 90 degrees clockwise, are centered.
  *
- * https://docs.microsoft.com/en-us/typography/opentype/spec/baselinetags
+ * Baseline tags from https://docs.microsoft.com/en-us/typography/opentype/spec/baselinetags
  *
  * Since: REPLACEME
  */