[Mimedefang] documentation bug/omission for newbies

[starlight.2016q2 at binnacle.cx]

Correction: "Guaranteed multipart" applies to only
filter_begin() not to filter*().  I suggest adding
brief documentation indicating the expected state of
$entity for each of the filter*() callbacks.

Some expansion regarding when filter() and
filter_multipart() are called is worthwhile.  Reading
mimedefang.pl leaves me unsure regarding exactly
what-is-called-when and I would have to spend a few
hours experimenting to fully grasp it.  Reading the
original presentation doc left be a bit fuzzy on this
while reading MIME-tools gave me the conviction to just
go straight to part[0] for my purpose.