Docs: Added Note about tags vs. version constraint

I made this mistake myself a few days ago, so I think it will be useful to note that here.

https://github.com/yiisoft/yii2/commit/85a15424bd1d84525bd05ddc19ff157f5cb0e829
https://github.com/yiisoft/yii2/commit/594fd2daed88ad939acf727467e8a4f4d3511f95