nodiscard for certain returned variables

Nobody has claimed this yet.

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

The issue names no files, tests, or entry points. Start by locating the handling for LuaDoc @return annotations and existing discard-related diagnostics, then clarify the intended scope and syntax for marking selected return values. Done means the chosen annotation behavior is implemented with coverage for functions returning multiple values.

Written by the indexing model from the issue text.