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.
wrap 0: [0, 'orig']
wrap 1: [1, 0, 'orig']
wrap 2: [2, 1, 0, 'orig']
wrap 3: [3, 2, 1, 0, 'orig']
wrap 4: [4, 3, 2, 1, 0, 'orig']
wrap 0: [0, 4, 3, 2, 1, 0, 'orig']
unwrap 3: 3: [0, 4, 2, 1, 0, 'orig']
unwrap -: 0: [4, 2, 1, 0, 'orig']
unwrap 0: 0: [4, 2, 1, 'orig']
unwrap 4: 4: [2, 1, 'orig']
unwrap 0: -: ValueError
unwrap 2: 2: [1, 'orig']
unwrap 1: 1: ['orig']
unwrap -: -: IndexError