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
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fd079193d24ce81fdc31fbba687cadc70609c44d

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