I'm concerned about the news section of tags, like on https://stackoverflow.com/tags/yii/info.
- Should it be there?
- If yes, is there a limit to how old the news can be?
- If yes, is there a limit to how many news there can be?
No. News is not useful for the Q/A format. Information should be as timeless as possible.
Introducing a news section would require extensive editing work to keep it up-to-date. It would also mean someone has to write articles or find useful sites to link to. Links go old or are prove to get spammy, so that would require even more work.
There are other sites good in showing news. SE is good in Q/A. Let's keep it that way.
This is really up to the active users of each tag.
Tag wiki is not limited by Stack Exchange. It can be only 1-2 lines explaining what is the tag about, and it can be full fledged wikipedia-like page with links, explanations, and everything.
Usually I'll say "the more the merrier", but as this other answer correctly mentions, more means more time will be needed to maintain this and in the future it might become just an old and outdated archive.
So my opinion: let the active users do as they wish. No need to write any policy. And if you are one of those active users, do as you wish, just keep it tidy and try to keep it always up to date.