clarify order of items in <listPerson>

[ingoboerner]

This is a yes/no/it depends question to @lehkost:

Just to make sure and have this somewhere as a point of reference:

The order of items (<person>, <personGrp>) of the <listPerson> is the order of appearance? Is this a MUST or SHOULD requirement or do we simply don't know how encoders handled that in the present. There might be a hint in the Wikis (I haven't checked explicitly though).

Will go into encoding guidelines in #67