Fix of doc for synchronous_standby_names.

Hello, now the synchronous_standby_names can teach to ensure more
then one synchronous standbys. But the doc for it seems assuming
only one synchronous standby.

> There is no mechanism to enforce uniqueness. In case of
> duplicates one of the matching standbys will be considered as
> higher priority, though exactly which one is indeterminate.

The patch attatched edits the above to the following.

> There is no mechanism to enforce uniqueness. In case of
> duplicates some of the matching standbys will be considered as
> higher priority, though they are chosen in an indeterminate way.

Is this makes sense?

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center