Correct stdin/stdout usage in COPY .. PROGRAM

The COPY documentation incorrectly stated, for the PROGRAM case,
that we read from stdin and wrote to stdout.  Fix that, and improve
consistency by referring to the 'PostgreSQL' user instead of the
'postgres' user, as is done in the rest of the COPY documentation.

Pointed out by Peter van Dijk.

Back-patch to 9.3 where COPY .. PROGRAM was introduced.

doc/src/sgml/ref/copy.sgml

@@ -71,12 +71,14 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
   <para>
    <command>COPY</command> with a file name instructs the
    <productname>PostgreSQL</productname> server to directly read from
-   or write to a file. The file must be accessible to the server and
-   the name must be specified from the viewpoint of the server. When
-   <literal>PROGRAM</literal> is specified, the server executes the
-   given command, and reads from its standard input, or writes to its
-   standard output. The command must be specified from the viewpoint of the
-   server, and be executable by the <literal>postgres</> user. When
+   or write to a file. The file must be accessible by the
+   <productname>PostgreSQL</productname> user (the user ID the server
+   runs as) and the name must be specified from the viewpoint of the
+   server. When <literal>PROGRAM</literal> is specified, the server
+   executes the given command and reads from the standard output of the
+   program, or writes to the standard input of the program. The command
+   must be specified from the viewpoint of the server, and be executable
+   by the <productname>PostgreSQL</productname> user.  When
    <literal>STDIN</literal> or <literal>STDOUT</literal> is
    specified, data is transmitted via the connection between the
    client and the server.