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

doc/src/sgml/ref/drop_subscription.sgml

@@ -84,9 +84,11 @@ DROP SUBSCRIPTION [ IF EXISTS ] <replaceable class="parameter">name</replaceable
    for the subscription on the remote host are released.  If this fails,
    either because the remote host is not reachable or because the remote
    replication slot cannot be dropped or does not exist or never existed,
-   the <command>DROP SUBSCRIPTION</command> command will fail.  To proceed in
+   the <command>DROP SUBSCRIPTION</command> command will fail.  To proceed
-   this situation, disassociate the subscription from the replication slot by
+   in this situation, first disable the subscription by executing
-   executing <literal>ALTER SUBSCRIPTION ... SET (slot_name = NONE)</literal>.
+   <literal>ALTER SUBSCRIPTION ... DISABLE</literal>, and then disassociate
+   it from the replication slot by executing
+   <literal>ALTER SUBSCRIPTION ... SET (slot_name = NONE)</literal>.
    After that, <command>DROP SUBSCRIPTION</command> will no longer attempt any
    actions on a remote host.  Note that if the remote replication slot still
    exists, it should then be dropped manually; otherwise it will continue to

src/backend/commands/subscriptioncmds.c

@@ -992,7 +992,8 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 						"drop the replication slot \"%s\"", slotname),
 				 errdetail("The error was: %s", err),
 		/* translator: %s is an SQL ALTER command */
-				 errhint("Use %s to disassociate the subscription from the slot.",
+				 errhint("Use %s to disable the subscription, and then use %s to disassociate it from the slot.",
+						 "ALTER SUBSCRIPTION ... DISABLE",
 						 "ALTER SUBSCRIPTION ... SET (slot_name = NONE)")));
 
 	PG_TRY();