Fix docs to not claim ECPG's SET CONNECTION is not thread-aware.

Changed by: Tsunakawa, Takayuki <tsunakawa.takay@jp.fujitsu.com>

doc/src/sgml/ecpg.sgml

@@ -240,8 +240,7 @@ EXEC SQL AT <replaceable>connection-name</replaceable> SELECT ...;
   <para>
    If your application uses multiple threads of execution, they cannot share a
    connection concurrently. You must either explicitly control access to the connection
-   (using mutexes) or use a connection for each thread. If each thread uses its own connection,
-   you will need to use the AT clause to specify which connection the thread will use.
+   (using mutexes) or use a connection for each thread.
   </para>
 
   <para>
@@ -251,7 +250,7 @@ EXEC SQL AT <replaceable>connection-name</replaceable> SELECT ...;
 EXEC SQL SET CONNECTION <replaceable>connection-name</replaceable>;
 </programlisting>
    This option is particularly convenient if many statements are to be
-   executed on the same connection.  It is not thread-aware.
+   executed on the same connection.
   </para>
 
   <para>