pgsql: Doc: Clarify interactions of pg_receivewal with remote_apply

Doc: Clarify interactions of pg_receivewal with remote_apply

Using pg_receivewal with synchronous_commit = remote_apply set in the
backend is incompatible if pg_receivewal is a synchronous standby as it
never applies WAL, so document this problem and solutions to it.

Backpatch to 9.6, where remote_apply has been added.

Author: Robert Haas, Jesper Pedersen
Reviewed-by: Laurenz Albe, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/1427a2d3-1e51-9335-1931-4f8853d90d5e@redhat.com
Backpatch-through: 9.6

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1c423463e4b91ec784b935895d77b6ceb3301141

Modified Files
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doc/src/sgml/ref/pg_receivewal.sgml | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)