update: fix typo/stale comment to match code
The comment about "obsolete.background" seems to have been about
obsolete.foreground ever since it was introduced in a59e575c6ff8
(update: allow dirty update to foreground (successors), 2013-04-16),
so let's change it.
#require docutils gettext $ $TESTDIR/check-gendoc ro checking for parse errors