[docs] A bit more buffers documentation
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index a28d47f..e24ffa4 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -115,7 +115,12 @@
hb_buffer_get_user_data (hb_buffer_t *buffer,
hb_user_data_key_t *key);
-
+/**
+ * hb_buffer_content_type_t:
+ * @HB_BUFFER_CONTENT_TYPE_INVALID: Initial value for new buffer.
+ * @HB_BUFFER_CONTENT_TYPE_UNICODE: The buffer contains input characters (before shaping).
+ * @HB_BUFFER_CONTENT_TYPE_GLYPHS: The buffer contains output glyphs (after shaping).
+ */
typedef enum {
HB_BUFFER_CONTENT_TYPE_INVALID = 0,
HB_BUFFER_CONTENT_TYPE_UNICODE,
