Add comments explaining the foreign object copying code
Add comments explaining the foreign object copying code

These are the comments I would have liked to have been able to read
while fixing #449 and #478.

jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf,jberkenbilt/qpdf,qpdf/qpdf