CDRIVER-3347 add with_txn example
CDRIVER-3347 add with_txn example

rcsanchez97/mongo-c-driver,mongodb/mongo-c-driver,beingmeta/mongo-c-driver,jmikola/mongo-c-driver,jmikola/mongo-c-driver,jmikola/mongo-c-driver,mongodb/mongo-c-driver,rcsanchez97/mongo-c-driver,jmikola/mongo-c-driver,mongodb/mongo-c-driver,mongodb/mongo-c-driver,mongodb/mongo-c-driver,jmikola/mongo-c-driver,beingmeta/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,rcsanchez97/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,mongodb/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,beingmeta/mongo-c-driver,mongodb/mongo-c-driver,beingmeta/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver