[Mimedefang] documentation bug/omission for newbies

[starlight.2016q2 at binnacle.cx]

This is helpful:

https://msdn.microsoft.com/en-us/library/office/aa563064%28v=exchg.140%29.aspx

Shows a MIME hierarchy with a picture!  Coming to this without having ever pondered MIME before (sorry) left me at a disadvantage when trying to write my first filter beyond cut/paste.  Perhaps a note to newbies that they should read briefly about MIME hierarchies is a good idea.