pgsql: Fix the errhint message and docs for drop subscription failure.

Fix the errhint message and docs for drop subscription failure.

The existing errhint message and docs were missing the fact that we can't
disassociate from the slot unless the subscription is disabled.

Author: Robert Sjöblom, Peter Smith
Reviewed-by: Peter Eisentraut, Amit Kapila
Backpatch-through: 11
Discussion: https://postgr.es/m/807bdf85-61ea-88e2-5712-6d9fcd4eabff@fortnox.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a734caa25f79b7bfa423dd97162e5ecf14d1feb5

Modified Files
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doc/src/sgml/ref/drop_subscription.sgml | 8 +++++---
src/backend/commands/subscriptioncmds.c | 3 ++-
2 files changed, 7 insertions(+), 4 deletions(-)