A client can send a MarkupKind[] capability for completion, signatureHelp and hover (search for MarkupKind[] string here), but as far as I can tell the current code ignores this and always responds with a "markdown".

https://github.com/mads-hartmann/bash-language-server/blob/ce72fbbba968c02ec4bf99673c160bb859c59527/server/src/server.ts#L138-L143