Minor LSP fixups

[tgolsson]

• A formatting error in a detail
• I started writing a rune mode for emacs (again...), and noticed that returning a u64 as the data is out of spec -- only u32/i32 are legal. This causes emacs to barf because the JSON parser there assumes i64 as the integer value. This data doesn't seem to be used, but at least this transform is reversible if we want it.
• I noticed that completing json:: didn't include json:: as the prefix, leading to very odd results. I think the symbol-heuristic is a bit too naive still, but this is a tiny bit better. For example, completing the example would lead to json::json::from_string, whereas now it'll correctly only have json::to_string

[udoprog]

Thanks for giving this some attention!