documentation/doxygen: properly handle <sect5> and <sect6> everywhere.

documentation/doxygen: properly handle <sect5> and <sect6> everywhere.

Insufficient testing in 2b57d535dfddbee13cb30ad352c42e68e7915553 and
before led to this (hitting the unreachable assert below), fortunately
looking at code coverage revealed the "more than three levels of
Markdown headings" message was never tested properly.