nodiscard for certain returned variables

還沒有人認領這個 Issue。

Functions that return multiple variables may have variables that are important or not. There should be a way to flag any return value as nodiscard.

```lua
---@return number index
---@return {number:string} list
---@nodiscardall
---@return number nonimportantvariable
function getREALLYImportantList()
```
I suppose nodiscardall that marks any return above it to be important.
Maybe even `@nodiscardabove` would workout

此 issue 未指定檔案、測試或進入點。先找出 LuaDoc @return 註解的處理方式，以及現有與捨棄相關的診斷，然後釐清標記選定回傳值的預期範圍和語法。完成的標準是實作選定的註解行為，並涵蓋回傳多個值的函式。

由索引模型根據 Issue 內容生成。