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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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0b79042701ff845b9a580443c822d52566c1ec24

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(-)