Doc: fix the note related to the GUC "synchronized_standby_slots"

Hi,

When I read the following documentation related to the "synchronized_standby_slots", I misunderstood that data loss would not occur in the case of synchronous physical replication. However, this is incorrect (see reproduce.txt).

> Note that in the case of asynchronous replication, there remains a risk of data loss for transactions committed on the former primary server but have yet to be replicated to the new primary server.
https://www.postgresql.org/docs/17/logical-replication-failover.html

Am I missing something? IIUC, could you change the documentation as suggested in the attached patch? I also believe it would be better to move the sentence to the next paragraph because the note is related to "synchronized_standby_slots.".

Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION