Clarify description of piggy-backing, including moving all the

Clarify description of piggy-backing, including moving all the
elementSetSmallerThanSomewhatColour options from the Result Set
Options table into the Connection Options table (since that's where
they need to be set: by the time you set them on a Result Set, it's
too late).  This emerges from the work to standardise ZOOM options
across bindings and implementations.