pgsql: Document a few caveats in synchronous logical replication.

Document a few caveats in synchronous logical replication.

In a synchronous logical setup, locking [user] catalog tables can cause
deadlock. This is because logical decoding of transactions can lock
catalog tables to access them so exclusively locking those in transactions
can lead to deadlock. To avoid this users must refrain from having
exclusive locks on catalog tables.

Author: Takamichi Osumi
Reviewed-by: Vignesh C, Amit Kapila
Backpatch-through: 9.6
Discussion: https://www.postgresql.org/message-id/20210222222847.tpnb6eg3yiykzpky%40alap3.anarazel.de

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/acdb523cb642e190e55719b8e4769a99fc767606

Modified Files
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doc/src/sgml/logicaldecoding.sgml | 76 +++++++++++++++++++++++++++++----------
1 file changed, 57 insertions(+), 19 deletions(-)