Fixes #4184: mentioned a safer way of using dummy cache that works in case of merging configs of advanced app

Fixes #4051: Renamed `yii\caching\GroupDependency` to `TagDependency` and added support for associating multiple tags to a single cached data item

fixes #4301

fix docs

It really should be: defining, attaching, using.

Not sure that I follow the last two paragraphs, but I may just be tired.

Already mentioned earlier in the text

Edited through “attaching”; added “event handler order” subsection

I think it should be: event handlers, attaching, triggering