docs: remove Atlas Cloud references and update editor usage

Author: hgmzhn

doc/USAGE.md

@@ -352,7 +352,8 @@
   - 移动：拖拽文本框
   - 旋转：使用旋转手柄
   - 变形：拉伸文本框角点
-  - 新建：在画布空白处右键，选择“添加文本框”
+  - 新建：在画布空白处右键，选择“添加文本框”，然后按住左键拖拽框出新文本框范围
+  - 框选：选择工具下，在画布空白处按住左键拖拽，可框选多个文本框
 
 - **文本编辑**：
   - 双击文本框修改译文

doc/en/USAGE.md

@@ -411,6 +411,8 @@ If you want to adjust the result after translation:
 - `No Selection`
 - `Brush`
 - `Eraser`
+- Right-click blank canvas space and choose `Add Text Box`, then drag with the left mouse button to draw the new text box.
+- With the selection tool active, drag from blank canvas space with the left mouse button to box-select multiple text boxes.
 
 Shortcuts: