LSP relatedInformation in loclist detail

## Information

**VIM version** Any

Operating System: Any

## What went wrong

Hello,
While writing my own LSP, I identified a bug:

In the function for handling LSP diagnostic response, the `diagnostic.relatedInformation` is parsed into `loclist_item.detail` in https://github.com/dense-analysis/ale/blob/fdadaed2ba93432add241bb25f9935dc2ebb4152/autoload/ale/lsp/response.vim#L58-L68
and then the `loclist_item.detail` is overridden with `diagnostic.message` if `diagnostic.source` exists
https://github.com/dense-analysis/ale/blob/fdadaed2ba93432add241bb25f9935dc2ebb4152/autoload/ale/lsp/response.vim#L70-L76

I suppose it would be better to reuse the previous `loclist_item.detail` instead of overriding it with message.
So my question is: is this really a bug or a wanted feature.
Depending on the answer I'll try to provide a PR with a fix.