[docs] add valid examples to guides/user/chapters/tags.html

[willianrampazzo]

The examples on https://github.com/avocado-framework/avocado/blob/5a15435a22ba49412bca5890e4e5028b552279b6/docs/source/guides/user/chapters/tags.rst are not executable anymore as the tests mentioned are not available on Avocado.

It would be good to create valid tests and update the whole tag's documentation.