Make it easier to see the "borders" of a tab

[shainaraskas]

When working with / reading tabs, it's hard to see where the contents of the tab end. it makes it difficult to see what instructions are specific to the tab context, and what instructions are not.

this is important for readers who are trying to jump to the "outside of the tab" info (e.g. someone who has already performed the tab steps)

it would be nice if the entire tab was in some sort of bordered container, or if there was a faint line at the end of the container that matched the tabbed interface border.

sometimes, tabs contain headings, so readers can't easily work around this by scanning for the next heading on the page.

[bmorelli25]

Partial implementation in #489.

[reakaleek]

#573

[shainaraskas]

fixed!