Update Forwarding Refs documentation to include use of keys

It's common practice in React to add keys to elements that are being iterated.
After looking carefully at the docs for forwardRef, and running into an issue with a key not being set, it appears that forwardRef returns an iterator, and requires a unique key is set on each element.
It would be great to have the docs for forwardRef reflect this requirement, and note anything unique e.g. common patterns to use when iterating to produce childComponents, which utilize forwardRef.

查看链接页面上的 forwardRef 文档，重点关注面向组件库维护者的注释，以及在迭代子组件时报告的行为。在使用相关指导和常见迭代模式更新页面之前，确认关键要求；完成的标准是文档清楚地解释 keys 如何应用于这些元素。