Doc: Update caveats in synchronous logical replication.
Reported-by: Simon Riggs
Author: Takamichi Osumi
Reviewed-by: Amit Kapila
Backpatch-through: 9.6
Discussion: https://www.postgresql.org/message-id/
20210222222847.tpnb6eg3yiykzpky@alap3.anarazel.de