Issue #61: document interaction between Transaction observers and save points.
Issue #61: document interaction between Transaction observers and save points.

groue/GRDB.swift,groue/GRDB.swift,zmeyc/GRDB.swift,groue/GRDB.swift,zmeyc/GRDB.swift,zmeyc/GRDB.swift,zmeyc/GRDB.swift,groue/GRDB.swift