Feature request: Analog of diff for custom declared function

Nobody has claimed this yet.

I want to write new declaration for some functions, for example, to add **generics** or some complex types (`@nodiscard` or even `integer`) that the wiki does not allow us to use.

However, if I do this in a "custom directory", then I completely replace the function description and eventually it becomes obsolete.

I would prefer to be able to write a "diff" when I can change the parameter types, but not change the parameter name/desc with a full method info.

```lua
--[[... unchanged info ...]]
---@generic T
---@param metaName `T` --[[... unchanged desc ...]]
---@return T? --[[... unchanged desc ...]]
---@nodiscard
function _G.FindMetaTable(metaName) end
```

Do you have any ideas on how to do this? I don't mean the real `diff` that is used in version control systems - maybe it's too complicated.

p.s. But yes, I don't want to write a `@nodiscard` to everything, it's stupid.

The issue names custom-directory declarations and the example function FindMetaTable, but no source files or tests. Start by tracing how custom declarations are loaded and merged with generated wiki data. Done would mean defining and implementing a selective override mechanism that can change types or add annotations while preserving unspecified names and descriptions.

Written by the indexing model from the issue text.