Improve comment for gncOwnerAutoApplyPaymentsWithLots

It was no longer accurate
Fix lot scrubbing for business lots

When adding splits to such a lot the code was supposed to
reduce splits in the same transaction and the same lot to
one single split. This is to avoid infinite fragmentation
over time.
That reduction wasn't working for business lots and should
be fixed with this commit.
This is also a potential fix for bug https://bugzilla.gnome.org/show_bug.cgi?id=741418
which could very well be a report of such infinite fragmentation.