[docs] Update hb-common documentation
diff --git a/src/hb-common.h b/src/hb-common.h
index 8a852e0..efe185c 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -136,8 +136,8 @@
/**
* hb_tag_t:
*
- * Data type for tag identifiers. Tags are arrays of four
- * bytes, each of which holds a character.
+ * Data type for tag identifiers. Tags are four
+ * byte integers, each byte representing a character.
*
* Tags are used to identify tables, design-variation axes,
* scripts, languages, font features, and baselines with
@@ -236,8 +236,8 @@
* HB_DIRECTION_IS_FORWARD:
* @dir: #hb_direction_t to test
*
- * Tests whether a text direction is forward. Requires
- * that the direction be valid.
+ * Tests whether a text direction moves forward (from left to right, or from
+ * top to bottom). Requires that the direction be valid.
*
**/
#define HB_DIRECTION_IS_FORWARD(dir) ((((unsigned int) (dir)) & ~2U) == 4)
@@ -245,8 +245,8 @@
* HB_DIRECTION_IS_BACKWARD:
* @dir: #hb_direction_t to test
*
- * Tests whether a text direction is backward. Requires
- * that the direction be valid.
+ * Tests whether a text direction moves backward (from right to left, or from
+ * bottom to top). Requires that the direction be valid.
*
**/
#define HB_DIRECTION_IS_BACKWARD(dir) ((((unsigned int) (dir)) & ~2U) == 5)
@@ -433,11 +433,9 @@
* @HB_SCRIPT_NYIAKENG_PUACHUE_HMONG
* @HB_SCRIPT_WANCHO
* @HB_SCRIPT_INVALID: #HB_TAG_NONE
- * @_HB_SCRIPT_MAX_VALUE: Dummy value
- * @_HB_SCRIPT_MAX_VALUE_SIGNED: Dummy value
*
* Data type for scripts. Each #hb_script_t's value is an #hb_tag_t corresponding
- * to the four-letter values defined at https://unicode.org/iso15924/ .
+ * to the four-letter values defined by [ISO 15924](https://unicode.org/iso15924/).
*
* See also the Script (sc) property of the Unicode Character Database.
*
