The implementation has always wrapped at 76 characters, rather than 72,
ever since it was introduced in commit 5cf8f1d4

 in 2006. At this
stage, it’s probably best to fix the documentation rather than the
implementation.

The likely bug in the implementation is the comparison
```
(++already) >= 19
```

19 × 4 = 76, so it seems like an off-by-one error in the comparison.
What was actually wanted was 18 × 4 = 72.

Thanks to Simon McVittie for the investigation and diagnosis.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1997