[i18n] Add translator notes (#5250)

Thanks to <https://github.com/koreader/koreader/pull/5237> we can now  extract the knowledge currently embedded in Transifex and put it directly in our source. This positively affects <https://github.com/koreader/koreader/issues/3754>.

Translation instructions and knowledge that comes out of localization-related questions should be preserved in the source, because Transifex is too ephemeral. For example, the links from <https://github.com/koreader/koreader/pull/2290> are no longer accessible. Even when they are, it's quite useful to have this information around while dealing with the code as well, and I also hope it'll be informative to contributors who seldom visit Transifex.

This commit also makes a few minor changes to obviate the need for comments where possible.

frontend/apps/reader/modules/readerfrontlight.lua

@@ -157,7 +157,7 @@ function ReaderFrontLight:onShowIntensity()
     if powerd:isFrontlightOff() then
         new_text = _("Frontlight disabled.")
     else
-        new_text = T(_("Frontlight intensity is set to %1."), powerd:frontlightIntensity())
+        new_text = T(_("Frontlight intensity set to %1."), powerd:frontlightIntensity())
     end
     UIManager:show(Notification:new{
         text = new_text,