data tree DOC mention binary value size units

Refs #1617
diff --git a/src/tree_data.h b/src/tree_data.h
index ed7b7da..fc60fd4 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -611,7 +611,7 @@
  */
 struct lyd_value_binary {
     void *data;     /**< binary value itself */
-    size_t size;    /**< size of the @p data value */
+    size_t size;    /**< size of the @p data value in bytes */
 };
 
 /**