doc: Document commands that cannot be run in a transaction block

Mainly covering the new CREATE SUBSCRIPTION and DROP SUBSCRIPTION, but
ALTER DATABASE SET TABLESPACE was also missing.

doc/src/sgml/ref/alter_database.sgml

@@ -164,6 +164,10 @@ ALTER DATABASE <replaceable class="PARAMETER">name</replaceable> RESET ALL
      <para>
       The new default tablespace of the database.
      </para>
+
+     <para>
+      This form of the command cannot be executed inside a transaction block.
+     </para>
     </listitem>
    </varlistentry>
 

doc/src/sgml/ref/create_subscription.sgml

@@ -48,11 +48,6 @@ CREATE SUBSCRIPTION <replaceable class="PARAMETER">subscription_name</replaceabl
    subscription at the commit of the transaction where this command is run.
   </para>
 
-  <para>
-   <command>CREATE SUBSCRIPTION</command> cannot be executed inside a
-   transaction block when the parameter <literal>create_slot</literal> is specified.
-  </para>
-
   <para>
    Additional info about subscriptions and logical replication as a whole
    can is available at <xref linkend="logical-replication-subscription"> and
@@ -227,6 +222,11 @@ CREATE SUBSCRIPTION <replaceable class="PARAMETER">subscription_name</replaceabl
    publication instance.
   </para>
 
+  <para>
+   When creating a replication slot (the default behavior), <command>CREATE
+   SUBSCRIPTION</command> cannot be executed inside a transaction block.
+  </para>
+
   <para>
    Creating a subscription that connects to the same database cluster (for
    example, to replicate between databases in the same cluster or to replicate

doc/src/sgml/ref/drop_subscription.sgml

@@ -93,6 +93,11 @@ DROP SUBSCRIPTION [ IF EXISTS ] <replaceable class="parameter">name</replaceable
    reserve WAL and might eventually cause the disk to fill up.  See
    also <xref linkend="logical-replication-subscription-slot">.
   </para>
+
+  <para>
+   If a subscription is associated with a replication slot, then <command>DROP
+   SUBSCRIPTION</command> cannot be executed inside a transaction block.
+  </para>
  </refsect1>
 
  <refsect1>