[doc] Tag @todo, @fixme and @warning (#5244)

This commit standardizes the various todos around the code a bit in a manner recognized by LDoc.

Besides drawing more attention by being displayed in the developer docs, they're also extractable with LDoc on the command line:

```sh
ldoc --tags todo,fixme *.lua
```

However, whether that particular usage offers any advantage over other search tools is questionable at best.

* and some random beautification

frontend/document/documentregistry.lua

@@ -105,7 +105,7 @@ end
 function DocumentRegistry:getProviders(file)
     local providers = {}
 
-    -- TODO: some implementation based on mime types?
+    --- @todo some implementation based on mime types?
     for _, provider in ipairs(self.providers) do
         local suffix = string.sub(file, -string.len(provider.extension) - 1)
         if string.lower(suffix) == "."..provider.extension then