TimedMediaHandler - missing groups

There are other messages and message groups that are translateable here, but still lack documentation.

Without repo access you should be able to add documentation via translatewiki too once the message group is re-enabled here. E.g. it was done here recently.

Date Diff was added 09/2015 (gerrit:241034). One requirement of translatewiki.net is existing message documentation, I do not know, why this was not done here. Maybe the description message is not so important.