Finish updating syncbase overview to reflect recent API changes.
Finish updating syncbase overview to reflect recent API changes.

We overhauled the Syncbase API recently.  This change finishes
updating the doc to reflect those API changes.

Change-Id: I8749b4b7885272e3a4c1ef77cdcd56e47cd3b44b

vanadium/website,vanadium/website,vanadium/website