Since the message might appear several times on the same page, it's
useful to give a bit more context. Besides, usability tests show that
when there's a group with no headings, there's no clear indication on
the page that the group is actually a group and not a heading.
We're also adding some emphasis to the group name in the "Showing
headings" message, to be consistent with the emphasis we've
added the the group name in the "No headings" message.
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