js: add comment for beforeTextChange

doric-js/src/widget/input.ts

@@ -81,6 +81,15 @@ export class Input extends View {
     @Property
     onSubmitEditing?: (text: string) => void
 
+    /**
+     * Called before text is changed
+     * @param editing: text already in box
+     * @param replacement: text which will replace part of editing
+     * @param start: the start index of replacing part
+     * @param length: the length of replacing part
+     * 
+     * @returns: true means the replacement will take effect, otherwise does not
+     */
     @Property
     beforeTextChange?: (change: {
         editing: string,