[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/apps/reader/modules/readerrotation.lua

@@ -23,7 +23,7 @@ function ReaderRotation:init()
     end
 end
 
--- @TODO: reset rotation on new document, maybe on new page?
+--- @todo Reset rotation on new document, maybe on new page?
 
 function ReaderRotation:onRotate(rotate_by)
     self.current_rotation = (self.current_rotation + rotate_by) % 360