How to display vocabulary terms and their descriptions with their tagged content in a view?
If you don't mind using some other contrib modules with views, Views Field View can be the best solution.

Create content views

First, create a content views to list the content title which contains the taxonomy term, add field Content: Title and add the taxonomy term field as contextual filter. (e.g. Content: Tags (field_tags) in screenshot)


  Use aggregation is to remove the duplicated content because same term can be attached on one content more then once.




Create taxonomy term views

Second, create a taxonomy term views to list the Taxonomy term: Name and Taxonomy term: Term description under a vocabulary, which is similar to the one you have created.


  BUT, you don't need to add the relationship Content with Term neither the field Content: Title




Field Taxonomy term: Term ID is used as contextual argument, so exclude it from display.



Attach views field view

Add a views field, select the taxonomy term view you created and place [!tid] as the argument of Contextual filters.



Result

All required data is ready now, you may need to override the views template files for modifying the output.



Performance


  This method involves multiple views, the performance may become an issue if there are many contents and terms. You should enable the time-based cache on the content views.