clarify the sequence of pages of MultiPageFold

MultiPageFold elements SHALL reference all BinderySignature elements
including this BinderySignature that are produced in common so that each
surface of the final folded signature contains more than one reader page. The
folded signature is cut perpendicular to the final fold and results in the number
of final BinderySignatures equal to the number of values specified in the
MultiPageFold elements. In case of a vertical final fold, the first final BinderySignature SHALL be placed at the bottom of the unrotated final folded signature as defined in table E.5 pagination schemes. In case of a horizontal final fold, the first final BinderySignature SHALL be placed to the left of the unrotated final folded signature as defined in table E.5 pagination schemes.Multiple identical MultiPageFold elements that reference
the same @BinderySignatureID SHALL be specified to indicate multi-up
production of the same BinderySignature. MultiPageFold SHALL NOT be specified
unless @BinderySignatureType="Fold". If MultiPageFold elements are
specified, they SHALL be specified in all BinderySignatures that are referenced
by MultiPageFold .
Note: Typical use cases are a final cut in a folding machine, coming and going
in book production or longitudinal slitting in a web press.