Describe assignment essentially

[SweetLoverFT]

As a cpp programmer, I think we should describe things essentially. The content from "2. storage赋值给memory，会创建独立的副本，修改其中一个不会影响另一个；反之亦然。例子：" should be collected into the content "4. 其他情况，变量赋值给storage，会创建独立的副本，修改其中一个不会影响另一个。"