When resolving documentation for code completion items, we fetch full
documentation through the newly added
`swiftide_completion_item_get_doc_full_copy` SourceKitD function, if not
found we fallback to brief documentation as before using
`swiftide_completion_item_get_doc_brief`.
> [!NOTE]
> Unlike brief documentation, SourceKitD doesn't cache full
documentation for completion results to avoid bloating memory with a lot
of large strings.
>
> As of now, SourceKit-LSP doesn't cache completion item documentation
either, should we introduce a new full documentation cache (e.g. using
`LRUCache`)?
