[clangd] Fix regression regarding new line handling for hover/signature help content (#162029)
Fix clangd/clangd#2513
This regression was introduced with #140498.
The issue is that with #140498 the extraction of the documentation
comment changed from line based to paragraph based.
This also removed some required line breaks inside paragraphs, which
used to be added before the change.
This PR adds the missing line breaks again.