Currently the the transclusion editor's list of templates includes /Doc and other subpages of templates that are not themselves actual templates.
At https://en.wikipedia.org/wiki/Wikipedia:VisualEditor/Feedback/Archive_2013_2#Removing_template_subpages_from_the_template_selection_list.3F there was a discussion about this which suggested possible ways forward:
- Exclude all subpages except those explicitly marked as templates, e.g by magic word or category
- Include all subpages except those explicitly marked as not being templates, e.g. by magic word or category