pgsql: doc: Add note about promotion waits on backends stuck sending to

doc: Add note about promotion waits on backends stuck sending to clients.

Commit df1be4ddc83 made promotion skip waiting for the barrier that
refreshes each process's cached logical WAL logging state in some
cases, but promotion still waits for it on a standby with wal_level =
replica that has a valid logical slot. A backend blocked sending data
to a client that has stopped reading can't process the barrier, so
promotion can wait for a long time.

Add a note about this case in the failover section, along with ways to
get out of the wait and to bound it in advance.

Backpatch to v19, where commit 67c20979ce7 introduced this wait.

Author: shihao zhong <zhong950419(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: shveta malik <shveta(dot)malik(at)gmail(dot)com>
Reviewed-by: Zhijie Hou <houzhijie22(at)gmail(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAGRkXqToBk9c2sPAgha-kCgxJhVwn90j1sCyUXrvydq51LKAqA@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f0d493168436eddbf24024437ddec253ca774e86

Modified Files
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doc/src/sgml/high-availability.sgml | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)