doc: update wording of ineffective SET and ABORT commands

Wording by Alvaro Herrera

doc/src/sgml/ref/abort.sgml

@@ -63,7 +63,8 @@ ABORT [ WORK | TRANSACTION ]
   </para>
 
   <para>
-   Issuing <command>ABORT</> outside of a transaction block has no effect.
+   Issuing <command>ABORT</> outside of a transaction block
+   emits a warning and otherwise has no effect.
   </para>
  </refsect1>
 

doc/src/sgml/ref/rollback.sgml

@@ -60,7 +60,7 @@ ROLLBACK [ WORK | TRANSACTION ]
 
   <para>
    Issuing <command>ROLLBACK</> outside of a transaction
-   block has no effect.
+   block emits a warning and otherwise has no effect.
   </para>
  </refsect1>
 

doc/src/sgml/ref/set.sgml

@@ -110,8 +110,9 @@ SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="PARAMETER">timezone</rep
      <para>
       Specifies that the command takes effect for only the current
       transaction.  After <command>COMMIT</> or <command>ROLLBACK</>,
-      the session-level setting takes effect again.  This has no effect
-      outside of a transaction block.
+      the session-level setting takes effect again.  Issuing this
+      outside of a transaction block emits a warning and otherwise has
+      no effect.
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/ref/set_constraints.sgml

@@ -99,7 +99,8 @@ SET CONSTRAINTS { ALL | <replaceable class="parameter">name</replaceable> [, ...
 
   <para>
    This command only alters the behavior of constraints within the
-   current transaction.  This has no effect outside of a transaction block.
+   current transaction.  Issuing this outside of a transaction block
+   emits a warning and otherwise has no effect.
   </para>
  </refsect1>
 

doc/src/sgml/ref/set_transaction.sgml

@@ -185,7 +185,7 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
   <para>
    If <command>SET TRANSACTION</command> is executed without a prior
    <command>START TRANSACTION</command> or <command>BEGIN</command>,
-   it will have no effect.
+   it emits a warning and otherwise has no effect.
   </para>
 
   <para>