[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/readerzooming.lua

@@ -116,9 +116,6 @@ function ReaderZooming:init()
 end
 
 function ReaderZooming:onReadSettings(config)
-    -- @TODO config file from old code base uses globalzoom_mode
-    -- instead of zoom_mode, we need to handle this imcompatibility
-    -- 04.12 2012 (houqp)
     local zoom_mode = config:readSetting("zoom_mode") or
                     G_reader_settings:readSetting("zoom_mode") or
                     self.DEFAULT_ZOOM_MODE