ds schema: reference and field-reference elements can interleave
ds schema: reference and field-reference elements can interleave

Also removed unnecessary minOccurs=1/maxOccurs=1 since that is the
default in schema.

osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi,osgi/osgi