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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6973868c9dbc8d6242a7419c0807d301dd6d5fe3

Modified Files
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doc/src/sgml/ref/drop_subscription.sgml | 8 +++++---
src/backend/commands/subscriptioncmds.c | 6 ++++--
2 files changed, 9 insertions(+), 5 deletions(-)