Document that COPY OUT requires an absolute pathname.

As suggested by Etsuro Fujita, but with somewhat different wording.

doc/src/sgml/ref/copy.sgml

@@ -116,9 +116,10 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
     <term><replaceable class="parameter">filename</replaceable></term>
     <listitem>
      <para>
-      The absolute path name of the input or output file.  Windows users
-      might need to use an <literal>E''</> string and double any backslashes
-      used in the path name.
+      The path name of the input or output file.  An input filename can be
+      an absolute or relative path, but an output filename must be an absolute
+      path.  Windows users might need to use an <literal>E''</> string and
+      double any backslashes used in the path name.
      </para>
     </listitem>
    </varlistentry>